Basic Definitions

acicular / radiating needles ~ crystals that grow in fine needles

adamantine ~ very shiny like a gemmy crystal, almost brilliant

agate ~ a waxy variety of cryptocrystalline quartz (chalcedony) in which the colours are in bands, clouds or distinct groups

alloy ~ a combination of metals to make a new metal compund

book ~ a stack of thin, flat crystals that form a "book", e.g. book of mica

botryoidal ~ looks like top of bunch of grapes

breccia ~ means angular, refers to a type of rock where angular pieces of rocks are “cemented” together with fine grained sand, type of sedimentary rock

button ~ a small cluster or plate of crystals, e.g. amethyst button

carat ~ a unit of weight for gems. A carat is one fifth of a gram (0.2g). (GS)

chatoyant ~ shines like a cat’s eye because of fibers

chemicals ~ everything on earth is made up of the 103 known chemical elements, including rocks, mineral, air, water, plants & animals

cleavage ~ the property to break along smooth lines or planes, the mineral has a shape it wants to be & breaks along those lines to keep that shape

common names ~ when rocks and minerals are known by names that are descriptive, not scientific

conchoidal ~ curved break like what happens with thick glass or bottle bottom, shell shaped, can be rough or smooth

concretion ~ spherical mass that is separate from the rock around it, usually weathers out of host rock, grows from the inside out

cryptocrystalline ~ when the crystals are so small, they cannot be seen except with a microscope

crystal habit ~ the form that the crystals of a mineral take when they have enough space and time to grow properly

crystal shape ~ the form or habit of a mineral, the shape that the mineral takes if it has the time & space to grow properly

crystals ~ minerals that form slowly have a distinctive crystal shape

cubic ~ 6 equal, square faces

dendritic ~ branching, tree-like growths

dimension stone ~ stones that are used for construction or architectural features are often described as dimension stones.

dog-tooth ~ shaped like the canine tooth, like a dog's tooth

dull / earthy ~ very dull, mainly in minerals that are porous

Earth's Crust ~ the earth's crust is made of solid, hardened rocks & minerals

erosion ~ the process through which mountains are broken down into boulders & sand

fibrous ~ used to describe mineral crystals that are long and thin and look like fibers

floaters ~ fully formed or doubly-terminated crystals that are unattached to other crystals

fossil ~ the remains of plants & animals that have been replaced by minerals

fracture ~ is the way a mineral breaks when it won’t break on a cleavage plane

gemmy ~ clear or transparent like a gem, used to describe transparency of a mineral

gemstones ~ rocks & minerals that have been cut & polished, used for decoration and are usually rare and valuable

geode ~ a sphere with a hollow inside, often lined with crystals, grows from the outside in

geologist ~ a scientist that studies rocks & minerals and the earth sciences

glassy ~ shiny like glass, found in 70% of minerals, vitreous

hardness ~ how easy it is to scratch a mineral

hexagonal prism with pyramid termination ~ hexagonal cross-section, with pointy ends

host rock ~ this is the rock that surrounds a mineral specimen, the matrix, aka country rock

igneous ~ rocks made from fire & heat, liquid magma that has cooled to form rocks

lapidary ~ the art of cutting, shaping, polishing and forming shapes, items, gemstones from rocks & minerals

luster ~ how shiny something is; words used to describe the way light reflects off of the surface of a specimen

massive ~ a mineral with no distinct crystal shape, large chunk of inter-grown minerals

matrix ~ the host rock that a mineral specimen or crystal is found in or on, bedrock

metallic ~ shiny like polished metal, highly reflective, usually opaque

metamorphic ~ igneous or sedimentary rocks that have been changed through extreme heat & pressure due to movement of the earth’s crust

mineral ~ non-living matter, chemically the same all the way through

minerals ~ all rocks are made of one or more of the 3000 known minerals

no cleavage ~ does not break regularly

nodule ~ rounded, spherical or kidney shaped rock or mineral, often with a "skin"; because it is harder than the surrounding rock it is in, it weathers out intact; e.g. pyrite nodule

opaque ~ cannot see through it at all, blocks all light, casts a solid shadow, acts like a wall

 ornamental stone ~ a rock or stone that is used for decoration, as an ornament - such as for lamps, book ends, wall and floor tile, tables, sculptures - usually because of its beautiful colour or pattern

paleontologist ~ a scientist who studies paleontology, learning about the forms of life that existed in former geologic periods, chiefly by studying fossils

pearly ~ like a pearl, play of colors on surface

physical properties ~ the common visible and tangible characteristics used in the identification & study of minerals

porphyry ~ a rock that is composed of angular pieces of rock "cemented" together - formed through the sedimentary process

prismatic ~ crystal shaped like a prism with flat sides and a pointy termination or end, e.g. quartz crystal

rhomb ~ looks like a squished box, short form for rhombohedral, resembling a prism with six four-sided faces

road cut ~ a road cut is where the rock has been cut to let the road go through, rather than making the road go up & down hills, rocks are cut to flatten the road

rock cycle ~ rocks are constantly forming, wearing down and forming again, very slowly however

rockhound ~ a lover of rocks, minerals & fossils who collects specimens in the field

rocks ~ non-living matter, made of 2 or more minerals

schiller ~ colors shimmer or flash when the light hits the surface in a certain way

sedimentary ~ layers of sand, clay & bits of rock laid down by water & turned to rock, often contains fossils

specific gravity ~ how heavy something feels when compared to what you would expect, heft, weight, mass, density

specimen ~ a term used to refer to a mineral or rock sample, a piece one is studying or using as an example

striations ~ parallel lines on the face of a crystal, looks like regularly spaced ridges or scratches

sub-metallic ~ soft shine like dull metal

tabular ~ divide easily into thin sheets, a stack is know as a “book”

termination ~ the point at the end of a crystal

translucent ~ see shadows and shapes through it when held up to the light, details not clear, is frosted or cloudy, like looking through wax paper or light

transparency ~ describes if you can see through something or not

transparent ~ clear, see through clearly all the way, like a plain window glass or clear plastic wrap, “gemmy” like a gemstone

uneven fracture ~ rough surface, not smooth

vesicle ~ a small circular enclosed space

vitreous ~ shiny like glass, found in 70% of minerals, glassy

vug ~ a hollow space in a rock where crystals often grow, e.g. Herkimer diamond in a vug

waxy ~ looks softly shiny like wax, like the surface of a wax candle

For a Geological Dictionary go to
For a Gemstone Glossary go to


1 The Earth's Crust

2 Rocks, Minerals

3 Cystals, Gemstones

4 The Rock Cycle

5 Erosion

6 Glossary of Terms