Most minerals are crystalline; they grow atom by atom, layer by layer, into a regular 3-D atomic structure. In most rocks, the crystals are too small to be seen without the use of a microscope, or the crystals are too small or irregular to be used as a gemstone. A few minerals are amorphous, which means that they have a weak crystal structure or lack one altogether. Crystalline materials may form with an irregular shape (massive), but there will be an internal regular 3-D atomic structure, even though no crystal faces are visible. The physical properties of crystals (their hardness and durability, the way they break or fracture) and their optical properties (the way light is reflected off surfaces or travels through them) are all related to the atomic structure.
The orientation of the flat faces of a crystal and their rate of growth control the final appearance of the crystal – its crystal habit. Most gemstones have a typical crystal habit, such as acicular (needlelike) crystals of rutile found as inclusions within quartz, amethyst prisms, spinel octahedral and fluorite cubes. Others have a number of habits
Geologists, mineralogist and gemologists use symmetry to divide crystals into different systems. Diamonds, for example forms as a structure of carbon atoms, bonded in such a way that cubes, octahedral (bipyramids) and other cubic crystals habits are created. Diamonds, like spinel and garnet, therefore belongs to the cubic system. There are several main crystal systems defined by their minimum symmetry. Cubic, tetragonal, hexagonal, trional, orthorhombic, monoclinic, triclinic.
Cleavage and fracture
A crystal can break along lines of weakness related to the atomic structure (cleavage) in such a way that a flat surface called a cleavage plane is left. Cleavage may be described as perfect (easy), distinct or indistinct. Where the break is not related to the atomic structure (fracture), the resulting surface will not be flat or smooth. Fracture may be described as uneven, splintery or conchoidal (the shell-like fracture of obsidian and many transparent gemstones).
Cleavage and fracture affect the strength of a gemstone and the ease with which it can be worked. Extra care must be taken when working with gemstones that have one or more directions of perfect cleavage. Topaz has three directions of perfect cleavage. Other gemstones with perfect cleavage include diamond, fluorite, calcite and spodumene (including kunzite).
The strength and durability of a gemstone is dependent upon its structure – on whether it is fragile and can be chipped, or is tough and fibrous – and its hardness. There are a number of scales used to compare the hardness of gemstones; the scale most commonly used by gemologists is the Mohs scale of hardness. Mohs arranged 10 common minerals in order of their “scratch ability” from the softest, at 1, which could be scratched by all those higher on the scale, to the hardest, diamond, at 10, which could scratch all others.