GEMSTONE : CRYSTAL SHAPES

Most mineral gemstone are crystalline, with their atoms arranged in regular and symmetrical patterns, like a lattice;a few are amorphous, with no or only a weak crystal structure. Crystalline minerals may consist of a single crystal, or of many in a group. Polycrystalline minerals are made up of many, usually small, crystals; in cryptocrystalline minerals the crystals are too small to see without the aid of a microscope.

Crystalline minerals are made up of a number of flat surfaces called faces; the orientation of these faces defines the overall shape, which is known as the "habit". Some minerals have a single. characteristic habit, such as pyramidal or prismatic; others may have several. A lump of crystalline mineral without a definite habit is called massive. Amorphous gemstones, like obsidian and tektites, have an irregular shape.

TWINNING

Natural crystals are seldom perfect. Their growth is influenced by external factors such as temperature, pressure, space, and the medium in which they grow. One irregularity that may occur is known as "twinning" - when the internal structure of the crystal is repeated. Twins grow together in a number of different ways and may show alternative colours.

GEMSTONES : PHYSICAL PROPERTIES

The physical properties of gemstones, their hardness, their specific gravity or density, and the way they break or "cleave", depend on chemical bonding and the atomic structure within the stone. For example, diamond is the hardest natural material known, and graphite is one of the softest, yet both are made of the same element, carbon. It is the way in which the carbon atoms are bonded together in diamond that gives it a reater hardness and resilience.

HARDNESS

One of the key qualities of a gemstone, hardness maybe measured by how well a stone resist scratching. Every stone can be tested and classified using the Moh's scale of hardness, which gives every mineral a figure from one to ten. Intervals between numbers on the scale are not equal, most obviously between nine and ten. Hardness testing is destructive, however, and should only be used on a gemstone if other tests fail.

SPECIFIC GRAVITY

The spesific gravity (SG) of a gem is an indication of its density. It is calculated by comparing the stone's weight with the weight of an equal volume of water. The greater a stone's specific gravity, the heavier it will feel. For example, a ruby with an SG 4.00 will feel heavier than an emerald with an SG 2.71 of similar size.

CLEAVAGE AND FRACTURE

Gemstones may break in two ways : they either cleave or they fracture. Which way they break depends on the internal atomic structure of the stone. Gems that cleave tend to break along planes of weak atomic bonding (cleavage planes). These planes are usually parallel, perpendicular, or diagonal to the crystal faces (as both planes and faces are directly related to the stone's atomic structure). A gemstone may have one or more directions of cleavage, which may be defined as perfect (almost perfectly smooth), distinct, or indistinct. Gems with perfect cleavage include diamond, fluorite, spodumene, topaz, and calcite.

When a gemstone breaks along a surface that is not related to its internal atomic structure, it is said to fracture.