Mineral Terms

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

accessory mineral - Mineral that occurs with other minerals.
acicular - An aggregate of long, slender, crystals. This term is also used to describe the crystal habit of single long, thin, slender crystals.
adamantine - Pertaining to luster. Transparent minerals with a very high luster are said to have an adamantine luster. 
aggregate - A grouping of crystals. Aggregates are defined by the ways crystals are clustered together.
alloy - A metal that is manufactured by combining two or more molten metals. An alloy is always harder than its component metals.
amber - Fossilized pine resin, many times used for ornamental purposes.
amorphous - Without a crystalline shape.
anisotropic - Lacking consistent hardness on all surfaces. For example, the hardness of Kyanite on the Mohs scale is between 6 and 7 lengthwise, and between 4 and 4½ crosswise
arragonite group - Group of minerals belonging to the carbonate group. They all crystallize in the orthorhombic system, exhibit good cleavage, have a weak double refraction in transparent specimens, and commonly intergrow in three individuals forming six sided trillings.
auriferous - Containing gold.
aventurization - The effect caused by small inclusions of a mineral with a highly reflective surface (commonly Hematite, Pyrite, or Goethite) which causes it to exhibit a glistening effect, as if it is pasted with glitter, when rotated or looked at different points. The name is derived from Aventurine, a green variety of Quartz that exhibits this effect.
axis - Imaginary line drawn through the center of an object, either horizontally or vertically. In the case of minerals, it is used to determine if and how mineral has symmetry. The horizontal axis is known as the x axis, the vertical axis as the y axis. Axis lines are usually drawn as dotted lines. Plural is axes.
axis of symmetry - An imaginary line drawn through the center of a crystal that replicates the exact shape if the crystal is turned to 360°. Symmetry can also be calculated on degrees, which depends on how often a crystal must be rotated to bring out symmetry.
banding - The presence color zoning lines, or "bands", in some minerals. Rocks with this characteristic are described as being "banded"
basal cleavage - Type of cleavage exhibited on a horizontal plane of a mineral by way of its base. Minerals with basal cleavage can sometimes be "peeled", like minerals of Mica group.
basalt - The dark, dense, aphanitic, extrusive rock that has a silica content of 40% to 50% and makes up most of the ocean floor. Basalt is the most abundant volcanic rock in the Earth's crust.
base - The top and the bottom "closing points" of a crystal. A base exists only in minerals that belong to the hexagonal, orthorhombic, tetragonal, and trigonal crystal systems. Matrix or foundation of a crystal.
bipyramid - Crystal shape in form with a plane dividing a crystal into two pyramids base to base.
bladed - Crystal habit describing flat, elongated, "knife-like" crystals. Like those of  Kyanite
botryoidal - Aggregate resembling a cluster of grapes. Also known as globular.
boule - Synthetic gems created from molten liquids placed in tear shaped molds to crystallize, leaving them with a tear-like form. Mostly applied to synthetic Rubies and Sapphires.
brass - Man-made alloy of copper and zinc. In the olden times, any alloy of copper was known as brass. Most brass of the olden days was an alloy of copper and tin.
brilliant - A type of cut used for certain gemstones or referring to luster.
brittle - Form of tenacity which describes a mineral that gets hammered and results in a fine powder or small crumbs. Minerals that are not brittle are referred as nonbrittle minerals. Brittle minerals leave a fine powder if scratched, which is the way to test a mineral to see if it is brittle.
cabachon - Gem or stone without facets that is highly polished and has smooth, rounded edges.
calcite (group) - Group of minerals belonging to the carbonate group that are isomorphous with one another and have the same properties.
capillary - Aggregate composed of tiny, thin, straight, long crystal strands; hair like.
carat - Weight measurement used in reference to gemstones in regard to their evaluation. A carat is .2 grams (or 200 milligrams), and this weight is used worldwide, even in the U.S. where the metric system isn't used. A point is the weight used only in reference to very small, precious gemstones, and represents 1/100th of a carat. The abbreviation for carat is Ct. and for point is Pt. The term carat in regard to gemstones should not be confused with the term carat in regard to gold. By gold, it refers to the content of gold a gold ornament contains. Because of the confusion, the term carat in regard to gold has been changed to karat.
carbonates (group) - Group of minerals that contain one or more metallic elements plus the carbonate radical (CO3). Most are lightly colored and transparent when pure. All carbonates are soft, brittle, and effervesce when exposed to warm hydrochloric acid. The carbonates are divided into Calcite Group and  Aragonite Group.
cavity - Hollow area in rock that develops because of some form of stress. Many cavities are lined with crystals. See also vug.
chatoyancy - Phenomenon of certain cat's eye minerals which causes it to exhibit a concentrated narrow band of reflected light across the center of the mineral.
chemical - A substance with a distinct molecular formation, produced by a chemical process.
chemical bond - A force by which atoms are bound in a molecule or crystal.
chemical formula - The scientific method of describing what elements a material is composed of.
chert - A member of a group of sedimentary rocks that consist primarily of microscopic silica crystals. Chert may be either organic or inorganic, but the most common forms are inorganic.
chromate (group) - Group of minerals that are compounds of one or more metallic elements combined with the chromate radical (CrO4). Minerals in this group are usually brightly colored and heavy. The chromates are rare minerals and are usually classified as a sub-group of the sulfates.
clast - Fragment of rock or mineral broken off from a large piece.
cleavage - The tendency of certain minerals to break along distinct planes in their crystal structures where the bonds are weakest. Cleavage is tested by striking or hammering a mineral, and is classified by the number of surfaces it produces and the angles between adjacent surfaces.
clinopinicoid - Describing an elongated crystal with a steep, slanted angle towards its top at the base. Minerals shaped this way are clinopinicoidal.
cluster - Dense agglomeration of crystals.
coating - A thin layer of one mineral on the surface of another.
color - The characteristic color or colors of a mineral.
concentric - Aggregate describing foliated masses that are somewhat spherical and rotate about a center; appearing like a rose (rosette). Also used to describe a form of banding where the bands are circular, forming rings about a central point.
concretion - Aggregate composed of a mass of small crystals that become cemented together, resulting in a rounded, odd form.
conglomerate - A rock composed of particles more than 2 millimeters in diameter and marked by the roundness of its component grains and rock fragments.
contact twinning - Form of twinning where two crystals join together at a base. Examples: japanese twin, spinel twin, and chrysoberyl twin.
coprolite - Fossilized animal excrement embedded in rock.
coral - Marine polyp that secretes calcareous skeletons. The skeleton is also called coral, and is used as an ornamental stone.
crazing - Condition in opal that causes it to form small, internal cracks, and in some severe cases will eventually disintegrate the opal.
cryptocrystaline - Composed of tiny, microscopic crystals.
crystal - A mineral in which the systematic internal arrangement of atoms is outwardly reflected as a latticework of repeated three-dimensional units that form a geometric solid with a surface consisting of symmetrical planes.
crystal form - The shape and habit of a particular crystal.
crystal lattice - The arrangement of atoms in a crystal, giving each crystal its distinct shape. See also crystal structure.
crystal structure - The geometric pattern created by the systematic internal arrangement of atoms in a mineral. 
crystal system - The primary method of classification of crystals. The Crystal system classifies crystals in six groups. They are: Isometric, Tetragonal, Hexagonal (which includes Trigonal), Orthorhombic, Monoclinic, and Triclinic. The crystal class, which classifies crystals into 32 crystal types, is a more precise classification of crystal groupings.
crystalline - Having a crystal structure / Composed of visible crystals.
crystallize - To form a crystal shape, or to have crystals in a particular group.
cube - Six sided polyhedron; all sides are equi-dimensional and bisect at 90º. Minerals shaped as cubes belong to the isometric system.
cut - A description of the type of facet.
 
deposit - An accumulation of certain minerals within a rock formation.
dichroi - Literally means "two colors". A mineral that exhibits one color when viewed from one angle but a different color when viewed from a different angle is said to display dichroism.
diffraction - The bending of light when it enters from one medium into another. For example, light bends as it travels from air into another substance, such as water. Diffraction also occurs when light enters from the air into a mineral, and the amount of diffraction varies among minerals.
dodecahedron - Twelve sided polyhedron; all sides are equidimensional and either rhombic or pentagonal. If the dodecahedron is composed of rhombs, it is known as a rhombic dodecahedron, or simply as a dodecahedron. If it is composed of pentagons, it is known as a pentagonal dodecahedron or pyritohedron.
double refraction - Phenomenon exhibited on all non-opaque minerals except for amorphous ones and ones that crystallize in the isometric system. A light ray enters the crystal and splits up into two separate rays, making anything observed through the crystal appear as double. The double refraction on most minerals is so weak that it cannot be observed without special instruments. However, in some minerals, such as the Iceland Spar variety of Calcite it is strongly seen. The double refraction is different in every mineral, and thus can be used to identify gems.
doublet - One of the many gem fakes in which a thin, flat section of a real gem is pasted atop a thick base of glass or rock crystal.
doubly terminated
druse - Aggregate composed of prismatic crystals protruding from a cavity or wall.
dull luster - The luster of minerals with very poor optical properties.
dusting - Very thin coating of one mineral on another mineral; a very thin sprinkling.
earthy luster - Luster describing minerals that are microcrystalline or amorphous and have very poor reflective surfaces.
effervescence - A chemical reaction where bubbles of gas escape a from a liquid, which is caused by two incompatible substances. Carbonate minerals effervesce if they come into contact with hydrochloric acid.
elongated - Describing a crystal with a lengthened side, meaning that one side is far longer than the other sides which are about equal.
embedded crystal - Crystal that is implanted in rock and can only be extracted if part of the rock is removed.
even fracture - Mineral fracture forming a smooth, flat surface.
face - An individual flat section of a faceted gem / crystal face.
facet - A desired surface displayed in a gem. It may grow naturally but is usually hand cut. This definition includes the meaning of a specific cuts for gems.
faceting - Cutting from a rough stone into a gem, creating a facet
feldspar (group)
Group of minerals that are aluminum silicates containing potassium, sodium, and/or calcium. This is the most abundant group of minerals on the earth, and the building block of many rocks.
fibrous - Aggregate describing a mineral constructed of fine, usually parallel threads. Some fibrous minerals contain cloth-like flexibility, meaning they can be bent around and feel like cotton.
filiform - Aggregate composed of thin, intergrown, twisted, pipelike strands.
flakey - Aggregate containing small, flat, almost bendable flakes.
flaw - Inclusion or crack in a gemstone that usually demotes its value.
fluorescence - Emission of visible light by a substance, such as a mineral, that is currently exposed to ultraviolet light and absorbs radiation from it. The light appears in the form of glowing, distinctive colors. The emission ends when the exposure to ultraviolet light ends. Minerals described as fluorescent have this ability.
fossil - Animal or plant remains of a previous age embedded and preserved in rock. Rocks containing fossils are described as fossiliferous.
fossilization - The process of an organic substance being transformed into a fossil.
fracture - A crack or break in a rock, to break in random places instead of cleaving, characteristic way a mineral breaks when put under stress, aside from cleavage.
friable - Describes a mineral which easily crumbles. Minerals with an earthy fracture are friable.
gem - A cut mineral or pearl refined for use as an ornament.
gemology - The science and procedures involved in mining, faceting, and marketing of gemstones. A person studying in this discipline is a gemologist.
gemstone - Any mineral or naturally occurring substance in an raw, uncut state that is capable of being a gem.
geode - Hollow rock that is filled or partially filled with crystals.
geology - The scientific study of the Earth, its origins and evolution, the materials that make it up, and the processes that act on it.
globular - The term globular is used as a synonym of botryoidal, but sometimes describes any rounded agglomeration, such as botryoidal, reniform and mammilary.
grain - Weight used in reference to measuring pearls. A grain is .05 grams or ¼ carat.
grainy - Crystal aggregate resembling a cluster of grain.
gram - Weight measurement used to measure less valuable gems or rough stones.
granular - Same as  grainy.
greasy (luster) - Luster of a mineral that appears coated with grease.
group
1) An aggregate of crystals. 
2) The classification order that minerals are arranged in based on their chemical structure. 
3) A scientifically recognized selection of minerals similar in structure. All groups have a specific group name, and individual minerals may also be categorized. A mineral in the group may have the same name as group name.
habit - The attributes of the appearance of a crystal or aggregate.
halides - Group of minerals containing one of the halogen elements (chlorine, fluorine, bromine, and iodine) as a building block. Most halides are soft and fragile, and some are soluble in water.
hardness - The degree of resistance of a given mineral to scratching, indicating the strength of the bonds that hold the mineral's atoms together. The hardness of a mineral is measured by rubbing it with substances of known hardness.
heat treated - A mineral or gem put under intense heat to enhance color or remove flaws.
hemimorphic -       doubly terminated crystal with two differently shaped ends.
host mineral - Mineral that is the chief constituent of a particular rock.
igneous rock - A rock made from molten (melted) or partly molten material that has cooled and solidified.
impurity - An item present in a mineral which is not part of its integral structure, and may change its optical properties, such as color.
inclusion - Materials that are locked inside a mineral as it is forming.
insoluble - Not able to be dissolved.
iridescence - Light effect causing a mineral to display a play of colors on an apparently single colored surface. Iridescence is many times the result of mineral inclusions which cause the effect.
ivory - Ornamental material made out of the calcareous teeth of many mammals, such as elephants, sea lion, hippopotamus, narwhal, and boar.
jagged fracture - Type of fracture resembling broken metal, exhibiting sharp, jagged surfaces.
jewel - A gem, whether cut or uncut, capable of being used as an ornament.
jeweler - Individual whose trade is to buy and sell jewels and gemstones.

karat - Unit of measurement describing the content of gold in an ornament. The karat unit measures the percentage of gold in metallic objects made of gold (mainly jewelry). Since pure gold bends too easily, it is mixed with tougher metals when made into ornaments to form a tougher, less pure, gold. The karat measurement determines the percentage of gold on a 1 to 24 scale, with 24 karats being pure gold. That means that 14 karat gold is only 14/24 (or 58.33%) gold. Gold with a high karat measurement is undesirable for jewelry because it is soft and bendable, and gold with a low karat measurement is undesirable because of its low gold content [hence lesser value]. The term karat can be confused with the term carat, which is the weight measurement for gemstones. The abbreviation of karat is k. or kt.
 
lattice - The arrangement of atoms in a crystal, giving each crystal its distinct shape.
locality - Area where a specific mineral was found or occurs.
loupe - Small magnifying lens used to observe certain aspects of a gemstone. Loupes usually have a magnification level of 10x.
luminesce - To give off light under certain conditions.
luminescence - General term describing the capability of a mineral with a habit of giving off light when put under certain conditions.
macrocrystalline - Having crystals large enough to be seen with an unaided eye.
magnetism - The property, possessed by certain materials, to attract or repel similar materials. Magnetism is associated with moving electricity.
malleable
Ability of a certain minerals that are able to be pounded into thin sheets.
matrix - A material that has an embedded crystal inside or emerging from it.
metallic luster - Exhibiting the luster of a metal, which is opaque and reflective. Some minerals exhibit a metallic luster even though they are not true metals.
mica - Individual member of the mica group. May also refer to any mineral with perfect basal cleavage that can be "peeled".
mica group - Group of related minerals that contain aluminum silicate. The percentage of its component elements may vary, which causes many people not to recognize the micas as a series. The micas are soft and have perfect basal cleavage, and can be seen when a mica is "peeled".
microcrystalline - Composed of tiny crystals that cannot be seen with an unaided eye. Microcrystalline minerals appear amorphous, since no apparent crystal shape can be detected.
micromount - A mineral specimen that is not more 1/10 of an inch (15 -27 mm.) in size.
mineral - Any naturally occurring, three dimensional, inorganic substance, with a chemical structure that can be exact, or can vary within limits. Elements that occur naturally are also listed as minerals.
mineral group - A scientifically recognized selection of minerals similar in structure. All groups have a specific group name, and individual minerals may also be categorized. A mineral in the group may have the same name as group name.
mineralogy - The study of minerals. Person who works in this discipline is a mineralogist.
Mohs hardness scale - A measurement that was devised by Austrian scientist Fredrick Mohs to determine the hardness of a mineral.
molybdates - These minerals are heavy, soft, and brittle. The molybdates are usually categorized with the chemically related tungstates, in which they can be partially replaced by.
monochromatic - Exhibiting only a single color. Minerals that are monochromatic occur in only one color, no matter what specimen.
monoclinic (crystal system) - Any mineral that falls under the following specifications belongs to the monoclinic crystal system: Three axes, all of them are unequal in length. Two of them are at right angles to each other, while the third is lies at an angle other than 90°.
mother-of-pearl - The iridescent inside of a mollusk shell, which is used as an ornament. Not to be confused with pearl.
nitrates group - These minerals are all fragile and soft. With one exception, they are all soluble in water, and are therefore found only in arid regions, primarily in dry lake deposits. The nitrates are a small group, and are sometimes classified as a sub-category of the carbonate group.
nodule - Aggregate consisting of a spherical lump, usually from groups of small crystals.
nonbrittle -  Do not crumbe to a powder when crushed.
non-crystalline - Not containing any crystals;  amorphous mineral or variety of mineral.
occurrence - The area where a particular mineral is found.
octahedron - Eight sided polyhedron; all sides are equidimensional and bisect at at the same angle. Minerals shaped as octahedrons belong to the isometric system.
opalescence - Effect seen in a few minerals, chiefly opal (hence its name) which cause it to exhibit a glimmer of different colors when rotated or seen in different angles. Opalescent describes mineral exhibiting this effect.
opalization - The common habit of Opal that it replaces material, such as wood, shells, and other minerals.
opaque - Not able to transmit light, which in effect disables it from letting an object be seen through it.
oxidation - The process of combining with oxygen ions. A mineral that is exposed to air may undergo oxidation as a form of chemical weathering.
oxides - Group of minerals that are compounds of one or more metallic elements combined with oxygen, water, or hydroxyl (OH). The oxide group contains the greatest variations of physical properties. Some are hard, some soft. Some have a metallic luster, others are clear and transparent. The Oxide group is divided into the Simple Oxides, Hydroxides, Multiple Oxides
oxidized - Having gone through a chemical change as a result of exposure to oxygen.
paleontology - The science and study of previous life forms, primarily in the form of fossils.
paramorph - A pseudomorph involving two minerals with an identical composition but different crystal structures. The original mineral forms, but conditions then cause it to be unstable, so it transforms into the other mineral with the same chemical structure while retaining the original crystal shape. An example of this is Aragonite that becomes unstable and transforms into Calcite but retains the original orthorhombic shape.
pearl - Smooth, round, shiny, organic object composed mainly of calcium carbonate found in the shells of some mollusks. Not to be confused with mother-of-pearl.
pearly luster - Exhibiting a luster similar to the inside of a mollusk shell or shirt button. Many mica's exhibit a pearly luster, and some minerals with a pearly luster have an iridescent hue. Some minerals may exhibit a pearly luster on cleavage cracks parallel and below the reflecting surface of a mineral.
penetration - The formation of a crystal penetrating through rock or another crystal.
petrified wood - Wood that is petrified, i.e. replaced by silica.
phantom growth - An interesting phenomenon exhibited when a crystal grows, than a new growth grows over the old crystal in the same direction, leaving an inscription of the previous growth on the crystal. Additional growth may be present, leading to the possibility of more than one phantom in a crystal.
polished - Either tumbled, faceted, or coated to enhance luster.
polymorph - A mineral that is identical to another mineral in chemical composition but differs from it in crystal structure.
porous - Containing rounded, tiny holes throughout. Many porous minerals can be dyed.
precious stone - Gem or gemstone that is highly appealing and very costly, exhibiting a powerful luster, high hardness, and rarity.
prism - Crystal that is elongated in one direction; the other directions are about equal.
prismatic - Crystal habit describing a crystal with four or more sides similar in length and width. Prismatic crystals are usually elongated in one direction.
prismatic cleavage - Type of cleavage exhibited on some prismatic minerals in which the mineral cleaves by breaking off thin, vertical, prismatic crystals off of the original prism.
pseudo - Assuming a false shape.
pseudomorph - One mineral that chemically replaces another mineral without changing the external form of the original mineral. There are three types of pseudomorphs: paramorphs, infiltration pseudomorphs, and incrustation pseudomorphs.
pure - Free of impurities.
pyramidal - Shaped as a pyramid.
pyroxene (group) - Group of minerals that contain iron, magnesium, silicon, and oxygen. They may also contain calcium, sodium, and lithium.
reflection - The event occurring when light hits a material and bounces off of it. There are different degrees of reflection, the strongest being light rays bouncing off a smooth, flat, polished surface, such as a mirror, where they bounce off and form a reversed image on the surface.
refraction - The bending of white light upon entering a new dimension, such as from air to water or from air to a crystal, and splitting the white light into the colors of the spectrum.
refractive index - The amount of refraction that takes place in a particular substance, which is a direct connection to the speed of light in that substance. The higher the refractive index, the greater the amount of dispersion, which increase the brilliance of a material.
repeated twinning - Form of twinning where two or more crystals form a repeated pattern.
resinous luster
Luster describing yellow, dark orange, or brown minerals with slightly high refractive indices (honey like).
reticulated - Aggregate composed of long crystals in a netting-like form, where all the crystals crisscross each other.
rhombohedral clevage - Type of cleavage exhibited on minerals that crystallize in the hexagonal system and as rhombohedrons, in which small rhombohedrons cleave off of the existing rhombohedron. Example is Calcite.
rhombohedron - Six sided polyhedron with each side shaped as a rhombus (an equilateral parallelogram).
rock - A naturally formed aggregate of an indefinite mixture of naturally occurring substances, mainly minerals. Its composition may vary in containment of minerals and organic substances, and are never exact.
rock crystal - Transparent, colorless, crystal of Quartz.
rock cycle - A series of events through which a rock changes, over time, between igneous, sedimentary, and metamorphic forms.
rough - Without any crystal faces. In regard to gemstones it refers to unfaceted material.
rutile twin - Two or more crystals that twin in a repeated pattern ("repeated twinning"). Named after the mineral Rutile,
secondary mineral - Mineral altered to a new form after undergoing a chemical change. An example is a mineral in the oxidation zone that transformed into another mineral through weathering.
setting - The proper placement of a gem on an ornament.
silica - Any material composed of only silicon and oxygen (and can include water), such as Quartz, Chalcedony, and Opal.
silica group - Group of silicate minerals (tectosilicates) composed only of silica (silicon dioxide -- SiO2 ). The minerals in this group are all the varieties of Quartz, Chalcedony, Opal, Tridymite, and Cristobalite (and a few rarer forms of silicon dioxide). Although Opal contains water in addition to silica, it is nevertheless in the silica group. The Silica Group is also known as the Quartz Group.
silky luster - Luster of minerals that have a very fine fibrous structure, causing it to display similar optical properties to silk cloth.
slab - Thick slice cut out of a solid mineral.
specific gravity - The ratio of the weight of a particular volume of a given substance to the weight of an equal volume of pure water.
specimen - A mineral or rock of interest to collectors or scientists.
spinel twin - Form of contact twinning, in which two octahedral crystals that twin at the base. Named after the mineral Spinel, which most frequently exhibits this form of twinning.
stone - Small piece of rock; may or may not refer to ornamental material.
streak - The color of a mineral in its powdered form. This color is usually determined by rubbing the mineral against an unglazed porcelain slab and observing the mark made by it on the slab.
streak plate - Unglazed piece of porcelain, such as a tile, used to test a mineral's streak
striated - Exhibiting tiny parallel lines or grooves.
subgroup - The next level of mineral classification after categorization in groups.
sub metallic luster - Luster of opaque to nearly opaque minerals with very good reflective properties.
synthetic   - Man made. Synthetic gems are created using molten chemicals to solidify and form the gem.
tabular - Crystal habit describing a flat, tough, usually four sided crystal.
tarnish - Property in certain minerals that cause them to discolor when placed in certain environments. An example is Silver, which develops a yellowish-black coating on its surface when exposed to hydrogen sulfide which is present in small quantities in the atmosphere.
termination - The end of a crystal face, usually referring to its base.
tetrahedron - Four sided polyhedron; all sides are equidimensional and triangular shaped. Minerals shaped as tetrahedrons belong to the isometric system.
texture - The feel and appearance of a mineral, such as color, crystal form, and luster.
transformation - The alteration of one mineral to another.
translucent - Describing something that is able to transmit light but not fully. Objects can be seen through a translucent object, but they will be unclear.
transparent - Describing an object that is able to transmit light through it, and if placed in front of another object, that object can be seen as if there is nothing interfering.
triboluminescence - Property exhibit in certain minerals which causes them to give off orange or yellow "flashes" when sawed or struck. 
triplet - Gem composed of three cross-sections pasted together. The top portion, which is the portion that is visible, consists of a naturally occurring, valuable gem, while the bottom two portions are low-grade material, such as glass or rock crystal.
tumbled - Having been placed in a tumbler, which smoothes out and enhances the luster of a mineral. Rocks and minerals may be naturally tumbled, if they were smoothed and rounded from having been in a fast-flowing stream or river for a long period of time.
tungstates group - Group of minerals that are compounds of one or more metallic elements. These minerals are very heavy, soft, and brittle.
twin - Two or more crystals that intergrow in a specific method. May also be used in verb form, as "Two crystals may twin to form a cross."
twinning - Tendency of some crystals to intergrow in a distinct way or form specific, repeated patterns.
ultraviolet light - Type of electromagnetic radiation which cannot be seen by humans. Ultraviolet, often abbreviated U.V. is term used when referring to ultraviolet light. Ultraviolet Lamp (U.V. lamp) is one that gives off ultraviolet light while blocking out white light, which causes fluorescent minerals to react.
uneven fracture - Fracture that leaves a rough or irregular surface.
variety - Titled variable of a mineral, characterized by a special property not present in other members of that same mineral such as color, location, chemical additive, crystal or aggregate, etc.
vitreous luster - Luster describing minerals with reflective properties similar to that of glass.
vug - Cavity in rock that is lined with long, slender crystals. A vug forms when air pockets form in cooling magma and allow crystals to form in the hollow area.

waxy (luster)
Luster of a mineral in which it appears to be coated by a layer of wax.

zeolite group - Group of about 30 minerals that are related in structure, habits, and occurrence. The zeolites are hydrous aluminum silicates that contain sodium or calcium, which can replace one another. 

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