Gem Stones, Fossils & Crystals
Gem stones and crystals that radiate with the light and energy of our Mother Earth.
Gemstones and crystals have been
a part of almost every culture on our planet. Some crystals and
gemstones have been used for worshipping, healing and even used by seers
to foretell the future. Most people are drawn to crystals and gem
stones for there beauty and mystic.
Understanding definitions and nomenclature concerning gem material
A Mineral
may be defined as a homogenous substance produced by the processes of
inorganic nature having a chemical composition and physical properties
which are constant within narrow limits. Its structure is crystalline.
It is composed of the same substance throughout.
Except for impurities it has the same chemical formula for all specimens
of the mineral. Its atoms usually have a definite and ordered crystal
structure. What makes a mineral (or an organic product) a gemstone is
cultural and partly subjective: beauty, durability and rarity.
Minerals often occur in geometrical forms bounded
by plane surfaces. These are crystals and the internal structure
determines properties which allow the identification of the gem material;
its differentiation from other minerals, imitations and sometimes
synthetics.
Crystals have:
1. An orderly and symmetrical atomic structure.
2. A definite external geometrical shape bounded by plane faces.
3. Physical (and optical) properties which vary with direction.
Glass has:
1. No regular atomic structure.
2. No tendency to assume a definite external shape.
3. Properties which are the same in all directions.
(the above derived from the British Gemological
Association’s course material. I strongly recommend their program to
aspiring gemologists)
Crystalline Material: Possesses the regular
structure and directional properties of a crystal but not the regular
geometrical shape. Also called massive. e.g. rose quartz.
Cryptocrystalline Material: Material which
consists of a multitude of tiny, often submicroscopic crystals. e.g.
Chalcedony.
Symmetry:
Crystal symmetry refers to the balanced pattern of the atomic structure
which is reflected in the external (crystal) shape. Different species vary
in the symmetrical arrangement of faces. These arrangements have certain
'planes' and 'axes' of symmetry. These form part of the definition of the
crystal system to which specific gemstones belong.
Plane of symmetry:
An imaginary plane dividing a body into two parts such that each is the
reflected image of the other. Crystals may have more than one plane of
symmetry. i.e. a cube has nine planes of symmetry.
Axis of symmetry:
An imaginary axis is placed through a perfect crystal so that during a
single rotation about this axis the outline of the crystal form appears
identically more than once; 2, 3, 4 or 6 times.
Center of symmetry:
(centrosymmetry)
Often present, it exists when every face of a perfect crystal is exactly
opposite a similar face on the other side of the crystal.
Crystal axes:
(Crystallographic axes)
To describe crystals imaginary lines are used intersecting at 0 (the
origin). These are specific to the various crystal systems, intersecting
at given angles and being of given lengths specific to each crystal
system.
Origin:
The intersection of the crystal axes.
Habit:
Gemstone species tend to occur in characteristic shapes which relate to
one or more of the forms common to the crystal system of the gemstone in
question. The crystal form or forms which a gemstone most often appear are
it's habit. e.g. diamond: octahedron, emerald: 6 sided prism.
Form:
Those faces of a crystal which are identically related to the crystal
axes. When the space so defined is completely enclosed (cube, octahedron)
it is a closed form. When identical faces do not completely enclose the
space (four or six sided prism; top and bottom open) it is an open form.
Twinned crystals:
(compound crystals)
A twin is a single crystal composed of two or more parts with any part in
reversed structural orientation to the next, or interpenetrated.
Contact twin:
Sharing a common plane.
Interpenetrant twin:
Two individuals have grown from a common origin and appear to penetrate
each other. e.g. cross stones.
Lamellar twinning:
A series of contact twins often as extremely thin plates. Atoms in
adjacent sheets are reversed, i.e. alternate plates are in the same order.
This can give rise to special optical effects as in the feldspar
labradorite.
Secondary twinning or
parting:
The crystal is composed of very thin plates parallel to definite
crystallographic directions. e.g. ruby, this gives rise to 'false
cleavage'.
CRYSTAL SYSTEMS
Cubic
Three crystal axes of equal length intersect at right angles to each
other. e.g. diamond, spinel, garnets. Tetragonal Three axes intersect at
right angles to each other. The vertical axis is of unequal length while
the two horizontal axes are of equal length. e.g. zircon, rutile.
Hexagonal
Four crystal axes. Three are of equal length and intersect at 60o to form
a horizontal plane which the fourth intersects at right angles. The
vertical fourth is of unequal length and forms an axis of 6fold symmetry.
e.g. Beryl, apatite.
Trigonal
Four crystal axes. Three of equal length intersecting to form a horizontal
plane which is intersected at right angles by the fourth axis. The
vertical fourth is of unequal length and forms an axis of 3fold symmetry.
e.g. quartz, corundum, tourmaline, dioptase, hematite.
Orthorhombic (Rhombic)
Three crystal axes of unequal length interest each other at right angles.
e.g. topaz, peridot, Chysoberyl, iolite, sinhalite, andalusite.
Monoclinic
Three axes. Two of unequal length intersect each other obliquely to form a
plane which is intersected by the vertical third (of unequal length) at
right angles. e.g. jadeite, nephrite, diopside, orthoclase feldspar,
serpentine, sphene, malachite, spodumene.
Triclinic
Three axes of unequal length intersect each other at oblique angles. e.g.
turquoise, labradorite.
NOTE: The
Gemological Institute of America (GIA)
system for crystal types differs from the above. Contact their web site
for information on their courses, books and so on.
CRYSTAL SYSTEM SYMMETRY
Singly Refractive: Amorphous  no crystal
structure



Optic Axis 
Cubic 
9 planes 
4 3fold 
 

13 axes 
3 4fold 
 

a center 
6 2fold 
 
Doubly Refractive:



Optic Axis 
Tetragonal 
5 planes 
1 4fold 
uniaxial


5 axes 
4 2fold 


a center 





Optic Axis 
Hexagonal 
7 planes 
6 2fold 
uniaxial 

7 axes 
1 6fold 


a center 





Optic Axis 
Trigonal 
3 planes 
3 2fold 
uniaxial 

4 axes 
1 3fold 


a center 





Optic Axis 
Orthorhombic 
3 planes 
3 2fold 
biaxial 

a center 





Optic Axis 
Monoclinic 
1 axis 

biaxial 

a center 





Optic Axis 
Triclinic 
no planes 

biaxial 

no axes 



a center 


Uniaxial
The optic axis of the crystal is parallel to the main crystal axis. One
direction of single refraction.
Biaxial
There are two directions of single refraction.
(optic axes)
GEMSTONES BY CRYSTAL
SYSTEM (major ones)
*Diamond simulants, manmade (U) = uniaxial, (B) =
biaxial
Cubic
Diamond
Sodalite
Fluorite
Spinel
GGG *
Strontium Titanate*
Garnet
Yttrium Aluminate*
Lazurite (Lapis Lazuli)
Yttrium oxide*
Pyrites
Cubic Zirconia*
Tetragonal (U)
Idocrase
Rutile
Zircon
Trigonal (U)
Calcite (marble)
Quartz
Corundum
Rhodochrosite
Dioptase
Tourmaline
Hematite
Orthorhombic (B)
Andalusite
Marcasite
Chrysoberyl
Peridot
Danburite
Sinhalite
Enstatite
Staurolite
Iolite
Topaz
Kornerupine Zoisite
Monoclinic (B)
Azurite
Nephrite
Diopside
Orthoclase Feldspar
Epidote
Serpentine
Euclase
Sphene
Jadeite
Spodumene
Malachite
Triclinic (B)
Axinite
Kyanite
Microcline Feldspar
Plagioclase Feldspar
Rhodonite
Turquoise
*Diamond simulants, manmade (U) = uniaxial, (B) =
biaxial
Minerals, Crystals and Their Systems
Copyright © Charles LewtonBrain 1986 1994
This is an introductory listing of definitions and nomenclature concerning gem materials.
