This article is an introductory listing of definitions and nomenclature concerning gem materials.
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.
(the above derived from the British Gemmological Association’s course material. I strongly recommend their program to aspiring gemmologists)
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.
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.
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.
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.
Often present, it exists when every face of a perfect crystal is exactly opposite a similar face on the other side of the crystal.
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.
The intersection of the crystal axes.
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.
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.
A twin is a single crystal composed of two or more parts with any part in reversed structural orientation to the next, or interpenetrated.
Sharing a common plane.
Two individuals have grown from a common origin and appear to penetrate each other. e.g. cross stones.
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.
The crystal is composed of very thin plates parallel to definite crystallographic directions. e.g. ruby, this gives rise to ‘false cleavage’.
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.
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.
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, haematite.
Three crystal axes of unequal length interest each other at right angles. e.g. topaz, peridot, Chysoberyl, iolite, sinhalite, andalusite.
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.
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.
Optic Axis  
Cubic  9 planes  4 3fold  
13 axes  3 4fold  
a centre  6 2fold 
Optic Axis  
Tetragonal  5 planes  1 4fold  uniaxial 
5 axes  4 2fold  
a centre 
Optic Axis  
Hexagonal  7 planes  6 2fold  uniaxial 
7 axes  1 6fold  
a centre 
Optic Axis  
Trigonal  3 planes  3 2fold  uniaxial 
4 axes  1 3fold  
a centre 
Optic Axis  
Orthorhombic  3 planes  3 2fold  biaxial 
a centre 
Optic Axis  
Monoclinic  1 axis  biaxial  
a centre 
Optic Axis  
Triclinic  no planes  biaxial  
no axes  
a centre 
The optic axis of the crystal is parallel to the main crystal axis. One direction of single refraction.
There are two directions of single refraction. (optic axes)
*Diamond simulants, manmade (U) = uniaxial, (B) = biaxial
Cubic
 Orthorhombic (B)

Tetragonal (U)
 Trigonal (U)

Monoclinic (B)
 
Triclinic (B)
