This article an introduction to Gemmology discussing the definitions concerning the physical properties of gemstones as written by Charles Lewton-Brain.
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
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.
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.
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.
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.
“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).
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 great deal.
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 plate.
A lapidary can make a set of small plates quite easily and synthetic corundum can supply the #9 plate.