Crystal Structure A perfect crystal is bounded by plane faces which meet at angles specific
for each kind of material (angle analysis can identify minerals). A crystal
may be cleaved in directions related to the external form or to a possible
crystal form for the mineral. Sometimes two distinct minerals can have the
same chemical composition with their differing properties being due to their
different crystal structure. Crystal structure affects mineral properties
more than their chemical nature. Examples here include diamond (carbon,
cubic) and graphite (carbon, hexagonal) and Calcite (trigonal) and aragonite
(orthorhombic), both forms of calcium carbonate. Properties Related to Crystal Structure
Optical: In the cubic system a light
ray is refracted (bent), passes through the crystal and emerges as a single
ray. This is known as an isotropic (singly refractive) material. Of the
doubly refractive crystal systems three (tetragonal, hexagonal, trigonal)
are uniaxial and have a single direction (not a line but an entire direction)
of single refraction in the doubly refractive (anisotropic or birefringent)
crystal. The orthorhombic, monoclinic and triclinic systems are biaxial
and have two directions of single refraction in the double refractive
(anisotropic or birefringent) crystal. In uniaxial crystals the isotropic
direction is that of the main crystal axis.
Pleochroism (Dichroism, trichroism): In doubly refractive gemstones the light ray is split and each part refracted
(bent) to a different degree. Assuming this ray is made up of white light
(which is composed of all colours) each ray has various colours absorbed
(filtered) so that each ray as it emerges from the gemstone is a different
(residual) colour. This is called dichroism (means two colours). Thus
depending upon the direction one looks at the stone relative to the crystal
and optical axes a different colour is seen. Both colours are often present
at the same time however and it requires a dichroscope to separate the
colours to see them. The dichroscope allows each ray's colour to be viewed
separately and at the same time to compare them.
Uniaxial gemstones are dichroic and two colours may be observed. Biaxial
stones are trichroic and three colours may be seen.
Heat Conductivity: Heat is conducted
differently in various minerals according to their crystal system. This
is used in Thermal Conductivity instruments to differentiate diamond which
conducts heat very well from its simulants and imitations. Some instruments
use it to identify other gemstones but they are expensive and of value
only when used with care and some gemmological knowledge. The use of standard
stones is suggested and drafts to be avoided as they can change the readings.
At its simplest this is the temperature test using tongue or lips for
glass and plastic.
Electrical Effects: Atomic structure
and the related crystal structure influence electrical properties. Some
crystals possess pyro-electricity. Tourmaline for example when heated
to between 100 - 100oC possesses polarity like a magnet needle. Another
effect of some polar crystals is piezo-electricity-pressure on a crystal
slab induces electrical charges on opposite faces. This is used in piezo-electric
gas lighters. If an alternating current is applied to the crystal it oscillates.
This is used in controlling radio wavelengths, usually using synthetic
quartz. Quartz watches use these properties. Silicon chips depend upon
the directional crystal properties to function. Electrical current is
conducted better in some gemstones than others. Natural blue diamonds
conduct electricity while the irradiated blue ones do not. A simple circuit
can be constructed to test this.
Cleavage: The is the tendency of
a crystallized mineral to break in definite directions related to the
crystal structure producing relatively smooth cleavage break surfaces.
Cleavage planes are always parallel to a particular cleavage face, i.e.
diamond cleaves in any of the four directions parallel to the faces of
the octahedron. Almost all crystals have a tendency to cleave. Those with
the least tendency to cleave include garnets, quartz, spinel (natural),
beryl and zircon. Gemstones with a strong tendency to cleave include diamond,
fluorite, topaz, peridot, kunzite (spodumene), euclase, sphene, axinite,
feldspars, synthetic spinel, dioptase and calcite.
Cleavage is described by the crystal face to which it is parallel; diamond
has octahedral cleavage, topaz has basal (parallel to the base of the
topaz crystal prism). The ease with which cleavage occurs and the resultant
smoothness of the cleavage break is described as perfect in topaz, indistinct
and difficult in beryl. Cleavage can be used in cutting diamonds and it
should be noted that stones with a strong tendency to cleave can be easily
cleaved in polishing and setting procedures.
Fracture: Defines the type of surface
obtained by breaking a crystal in a direction other than that of cleavage.
Types include conchoidal, shell-like as in glass and often in gemstones.
Also even, uneven and hackly or splintery as in nephrite. Identification
applications of cleavage/fracture include: Nephrite cleavage cracks occur
as 124o and jadeite at 93o.
Synthetic spinel imitating aquamarine may show cracks at right angles
and aquamarine does not.
Feldspars cleave and chalcedony does not. Tiny chips or breaks on the
girdle of cabachon feldspars (sunstone, moonstone, amazonite, etc.) are
flat and have a vitreous lustre while in chalcedony they are conchoidal
with a waxy lustre.
Splintery fracture is seen in nephrite and hematite.
Hematite fracture is splintery and hematite (a substitute) is not.
Conchoidal fractures are a strong indicator of glass. I've seen quartz
do it too to some degree.
Hardness: "The power a stone possesses
to resist abrasion when a pointed fragment of another substance is drawn
across its smooth surface without sufficient pressure to develop cleavage"
(GA course material).
Harder stones will scratch softer ones. Stones of the same hardness
may scratch each other (a diamond can scratch a diamond). The Mohs scale
is used for gemstone hardnesses. This scale is purely relative as shown
by the fact that the difference in hardness between corundum (9) and diamond
(10) is 140 times the difference between talc (1) and corundum (9).
- Orthoclase feldspar
Other reference points include:
- Finger nail 2 1/2
- Copper penny 3 or so\Window glass 5 1/2 or so
- Knife blade 6
- Steel file 6 1/2 - 7
- Silicon carbide 9 1/4
- Carborundum 9 1/4
Hardness testing is not often used as the chance of damaging a good
stone or even an imitation of value to the owner is too high. It is normally
only used on rough material or on an inconspicuous spot on large carvings
as a confirmatory test.
Any scratch detracts from the value of a gem. It will not tell if something
is synthetic or natural.
Hardness points Sets of standard pieces of Mohs hardness 7, 8, 9, 10
mounted in rods used to scratch gem materials.
Hardness Plates Sheets or slabs of standard hardness materials. The
gem to be tested is rubbed on the plate using the girdle so that hopefully
the plate suffers the damage. Again, material can scratch itself although
it is true that the feel of the "bite" in hardness testing can tell a
It is also not necessary to file chunks from gems or scratch whole facets;
a 1 mm scratch can suffice and if the plate and stone is wiped clean and
inspected with a loupe one can tell which was scratched. Diamond is the
only colourless gemstone which will produce a scratch in a polished corundum
A lapidary can make a set of small plates quite easily and synthetic
corundum can supply the #9 plate.