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 gem
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crystals,
gems and minerals
mineral glossary of terms
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Absorption Spectrum |
Colors of light least absorbed combining to produce
the color of the stone. The stone, when viewed by spectroscope, will
show as dark bands in characteristic positions the colors most
strongly absorbed. |
|
Acicular |
Needlelike; refers to the growth of a mineral in long
and slender crystals. |
|
Adamantine |
Very
high luster. |
|
Aggregate |
Intergrowth of several crystals, these may be
globular, fibrous, reniform, or radiating fibrous. |
|
Adularescence |
The
sheen of color seen in moonstone and other feldspars
of the adularia variety. |
|
Allochromatic Minerals |
Minerals that are colorless when pure, the color
coming from coloring agents, most of which are,
cobalt, copper, chromium, titanium, vanadium,
manganese, and iron. Examples of this are beryl,
corundum, quartz, and spinel. |
|
Amorphous |
Has
no characteristic external form or shape. The
arrangement of the atoms and molecules are irregular. |
|
Amphiboles |
A
group of closely related, dark colored rock forming
silicate minerals, as in, actinolite, hornblende. |
|
Anisotropic |
When
applied to crystals it is the display of unequal physical properties
in different directions. An example would be a mineral which has a
different hardness when tested in different directions. |
|
Anisotropic |
Crystals in which the optical properties vary with
direction. All crystals except those in the cubic system are in this
category, and exhibit double refraction. |
|
Aphanitic rock |
The
rock in which the crystals are too small to be seen by the unaided
eye. |
|
Asterism |
Stones containing suitably oriented rod like
inclusions or channels, that are cut as cabochon in the correct
direction show this star effect. Star effect. |
|
Atomic Weight |
Weight of an atom compared with an atom of oxygen
(16.00). |
|
Biaxial |
Two
optic axes or double refraction. Usually crystals in
the rhombic, monoclinic, and triclinic system. |
|
Birefringence |
Same
as double refraction. Splits rays of light passing through a
transparent object as glass or crystal. |
|
Botryoidal |
Resembling a bunch of grapes in rounded masses of a
mineral. |
|
Brilliant |
The
cut of a gemstone that is round and has 32 facets plus
the table above the girdle, (crown), and 24 facets
plus any culet below the girdle, (pavilion). |
|
Cabochon |
The
cut of a gemstone that has a convex surface. A cab. |
|
Cameo |
(Italian)
name for a relief that is cut so that it is raised. |
|
Chalcedony Species |
Moh's hardness: 6.5 - 7
Chemical composition: SiO2, silicon dioxide
Crypto crystalline quartz (microscopically small
crystals) generally known as chalcedony; includes
agate, petrified wood, chrysoprase, bloodstone,
jasper, carnelian, chalcedony, and sard. |
|
Chatoyancy |
Cat's eye effect produced by some gemstones when cut
properly in cabochon. See asterism. |
|
Chelsea Filter |
A
dichromatic color filter transmitting light of only
two wavelengths, one deep red the other yellow green.
Used to discriminate between emerald and synth. spinel
and green glass colored with cobalt. |
|
Cleavage |
The
tendency of stones to split along one or more definite directions,
always parallel to a possible crystal face. |
|
Conchoidal |
(Fracture), A breakage which leaves a conchoidal shell
shaped surface. |
|
Concretion |
Knobby or rounded mineral concentrations in
sedimentary rocks that are completely surrounded by rock. |
|
Crypto crystalline |
The
structure of a substance as chalcedony, that consist
of very small crystals but show no external sign of
crystal structure. |
|
Crystal |
A
homogeneous body in the form of a geometric solid bonded by
polyhedral faces, the nature of which is expression of the orderly
and periodic arrangement of its constituent atoms. |
|
Crystal Axes |
"Lines" passing through a crystal in important
symmetric directions, intersecting at the center of the crystal. |
|
Crystal Systems |
The
six main groups into which crystals can be classified:
triclinic, monoclinic, orthorhombic, cubic, tetragonal
and hexagonal. |
|
Dendrites |
Skeletal crystals that develop from supersaturated
solutions, often in small cracks, often resembling
plant or trees. |
|
Density |
The
ratio of the weight of a substance to its volume
expressed in g/cm 3, and numerically equal to the
specific gravity. |
|
Diaphaneity |
Showing light through its substance; transparent;
translucent. |
|
Dichroism |
Possessing the property of showing two different
colors when viewed from different angles. |
|
Diaphaneity |
Showing light through its substance; transparent;
translucent. |
|
Dispersion |
The
separation of white light into its constituent colors
by its refraction or diffraction. |
|
Double Refraction
|
Ability of certain crystals to split incident light
into two rays with different refractive indices. |
|
Dripstone |
Stalagmites or stalactites. |
|
Druse |
A
crystal coated surface of rock. |
|
Enhydro |
A
variety of chalcedony or or quartz species having the
center cavity filled with water. After removing the
mineral from the surrounding rock, the water often
dries out. |
|
Facet |
Man
made flat part of a mineral, a planar surface. |
|
Ferromagnetism |
Magnetic even in the absence of an external magnetic
field. |
|
Fire |
Same
as dispersion. |
|
Fluorescence |
A
mineral's ability to glow in the presence of
ultraviolet light. Some minerals will glow with
characteristic colors in long wave or short wave UV
light. |
|
Foliated |
Made
up of thin leaves, like mica. |
|
Fracture |
A
break with an uneven or irregular surface. |
|
Fraunhofer Lines |
A
series of groups of dark lines in the spectrum of an
object, visible using a spectroscope. |
|
Friable |
Easily crumbled or pulverized. |
|
Geniculated
|
Knee
like intergrowths of crystals. |
|
Geode |
A
stone having a cavity lined with crystals; the cavity
in such a stone. |
|
Girdle |
The
wide part of a cut gemstone. |
|
Gliding plane |
A
crystal direction along which the atoms can slip a
defined distance without destroying the coherence of
the crystal. |
|
Habit |
Characteristic crystal form. |
|
Hacklt |
A
fracture characteristic of metals in rock, like gold
and copper. (hackly) |
|
Hardness |
A
mineral's resistance to being scratched. The Mohs Hardness Scale
is most often used. It runs from 1 to
10, 10 being the hardest. The scale is not
linear, which is to say that hardness 10 is
not twice as hard as hardness 5. Each
hardness is represented by a particular
mineral, some by a common material, and the
actual hardness is indicated by the Rosiwal
cutting hardness:
H Mineral Common item Rosiwal cutting
hardness
__________________________________________
1 talc 0.03
2 gypsum fingernail 1.25
3 calcite copper coin 4.5
4 fluorite 5.0
5 apatite knife 6.5
6 orthoclase 37
7 quartz steel file 120
8 topaz 175
9 corundum 1,000
10 diamond 140,000 |
|
Hemimorphic |
Half
formed crystals in which the faces that grow on one
end are different in angle and position from the faces
to be found on the other end. |
|
Idiochromatic |
Minerals in which the color is due to an essential
constituent. |
|
Imitation Stones |
Substances used to look like a genuine. ie: glass,
plastic, etc. |
|
Inclusions |
Solid, gaseous, or liquid material of various types
incorporated in a crystal during its formation and
growth. Substances within a mineral, example, other
minerals, gas bubbles, liquids, or other foreign
objects. |
|
Intaglio |
(Italian) name for
engraving with a negative image, as used for
seals. |
|
Intarsia |
Mosaic inlay work using colored stones. (and wood) |
|
Labradorescence |
Patchy or diffuse iridescence due to the interference
of light by reflection from parallel inclusions.
Example Labradorite. |
|
Luster |
How
the mineral reflects light. It is described as
metallic, vitreous (glassy), waxy, greasy, silky, etc. |
|
Malleable |
Can
be flattened by pounding, as in metals. |
|
Mammillary |
Rounded mineral surface. |
|
Meteorite |
Solid object that comes from outer space and falls to
earth. |
|
Moh's Scale |
The
relative scale of the hardness of minerals, from 1 to
10, the order of hardness with no significance to
quantitative relationship. (See hardness) |
|
Nicol Prism |
Prism for producing polarized light, having split the
light into two rays. |
|
Nodule |
A
lump or node. |
|
Opaque |
Not
transparent or translucent. (You can't see through it) |
|
Optic Axis |
The
direction of single refraction in a double refractive
mineral. |
|
Optic sign |
If
the lower index of the refractive reading does not
vary when the gemstone is rotated, it is uniaxial and
+ optic sign, if the higher index is steady and the
lower index varies the optical sign is -. |
|
Ore |
A
mineral occurring in sufficient amounts to permit its
recovery at a profit. |
|
Organic gemstones |
This
group includes materials that, even though they are of
organic origin, have preserved or acquired a certain
stone character. They are an important part of the
trade, especially with respect to amber or pearl. Many
are made up of minerals; pearls are mostly aragonite,
bone is largely apatite. |
|
Orient |
Characteristic sheen and iridescence, as displayed in
a good pearl. |
|
Paste |
Lead
glass imitation stone. |
|
Pegmatites |
A
very coarse platonic rock, generally granitic in composition.
Usually forming dikes that cut granite or
the gneisses and schists that border granite
masses. They are coarse because the liquid
residue at the time of their crystallization
contained a high percentage of water and
other volatile elements that did not go into
the makeup of the common minerals of
granite, and were concentrated in the
residue. |
|
Phosphorescence
|
Fluorescence that continues for a while (however
short) after the ultraviolet light source is turned
off. |
|
Piezoelectric |
A
substance that becomes electrically charged by
pressure. |
|
Pleochroism |
A
phenomenon of certain minerals whereby the same stone
will exhibit different colors when viewed from
different angles under the same light source. Dichroism indicates two colors will be seen,
trichroism indicates three colors. The
effect is the result of different light
absorption patterns along different axes of
the crystal. |
|
Polarized Light |
Light which vibrates in one direction or plane. |
|
Potch |
Native material in or around a gemstone, a dead spot
or area. ie: ironstone surrounding opal. |
|
Quartz
Species |
Moh's
hardness: 7
Chemical composition: SiO2, silicon dioxide
Macro crystalline quartz (crystals
recognizable with the naked eye) includes
stones gemologists classify as varieties of
the quartz species: amethyst, aventurine,
rock crystal, blue quartz, citrine, hawk's
eye, prasiolite, quartz cat's eye, smoky
quartz, rose quartz, and tigereye. |
|
Radioactive |
Emitting alpha, beta, or gamma rays. |
|
Reconstructed Stones |
Old
method of fusing Stones made from chips. Used now to
describe amber re-melted and fused from otherwise
unusable pieces. |
|
Reflection |
The
throwing off or back, light from the surface. |
|
Refraction |
The
bending of light passing through one medium to
another. The changing of direction. |
|
Refractive Index |
The
measure of how much light is bent as it passes through
the material. Technically, it's the ratio of light ray
angles entering, passing through and exiting the
mineral. Values for natural gem materials range from
about 1.4 to 3.2. Generally speaking, the higher the
value the greater the potential sparkle in a stone (if
cut well). |
|
Refractometer |
Instrument used to determine the refractive index of
various substances. |
|
Rock |
Stony matter. Any mass of mineral matter forming an
essential part of the earth's crust. |
|
Sclerometer |
Device for the quantitative measuring of hardness. |
|
Sedimentary Rocks |
Formed
originally of sediment, including shale and sandstone,
composed of fragments of other rocks deposited after
transportation from their sources, and including those
formed by precipitation, as gypsum, or by calcareous
secretions of animals as in certain limestones. |
|
Sheen |
The
iridescence of light reflected from the surface of a
stone. ie: moonstone |
|
Specific Gravity |
The
weight of a substance compared to the weight of an
equal volume of pure water at 4 degrees C. |
|
Spectroscope |
Instrument which resolves light into its component
wavelengths by refraction through prisms or
diffraction grating. |
|
Stalactites, stalagmite |
The
formation of a cone-like structure that grows from the
deposits of carbonate of lime build up. |
|
Star |
The
effect of proper cutting of a gemstone as a cabochon
when the gemstone contains long rod like crystals or
fibers in parallel or cavities where these have
formally been. The reflection, (using a single light
source), from the surface produces lines in even
numbers, i.e.: 2,4,6,8,12, with a common center. |
|
Streak |
The
color of the powder from a mineral made when the
mineral is dragged across a piece of porcelain. The
trail, or streak, of powder can be a definitive color
in some cases (e.g. hematite's streak is red). |
|
Synthetic stones |
Man
made stones that have approx. the same composition and
or crystal structure of the natural crystal that they
represent. |
|
Transparency |
The
degree to which light passes through a substance. |
|
Twin Crystals |
Two
or more crystals which have grown together in a
symmetrical fashion. |
|
Uniaxial |
Minerals having single refraction, one optic axis. |
|
Vectorial properties |
Properties
of a mineral that vary with direction. |
|
Vein |
A
more or less upright sheet deposit of minerals,
cutting other rocks and formed from solutions rather
than from a molten magma as in a dike. |
|
Vicarious elements |
Those that take each other's place in trace amounts in
a minerals crystal structure. |
|
Vitreous |
Glasslike in appearance or texture. |
|
Vug |
An
open cavity in rocks, often lined with crystals. |
|
Xenolith |
Rock
enclosed in magma. |
|
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