The study of crystals, crystallography, can be a daunting subject at first, but it is an essential part of the understanding of gemstones. Here I would like to provide you with a little taste of the world of crystals.
Atoms are perpetually mobile, even in solid substances. This motion can be amplified by energy increases, such as temperature, causing atoms to transition from solid to liquid to gas. As pure substances harden, their atoms or ions form a crystal structure - solids with regular plane surfaces. Crystal characteristics, including shape and angular relationships, are identifiers for substances. Gemstones, typically existing as crystals at room temperature, are primarily minerals. Crystallography, the study of crystal formation and structure, plays a key role in understanding the property variations of crystalline gemstones.
Crystals are solid matter that have an orderly internal atomic structure which are bounded by flat surfaces. They are formed in many ways but the main 3 are:
· grown as magma cools,
· precipitation from water (opal),
· chemical reactions.
Historically, Hauy and Bravais contributed significantly to our understanding of crystals, with Hauy suggesting integrant molecules as constituent units and Bravais identifying 14 possible atomic arrangements or "space lattices". These lattices are the building blocks of all known crystal structures, which can be classified into seven crystal systems based on symmetry. The regular lattice structure in crystals was confirmed by Von Laue’s X-ray experiments in 1912, while W. H. Bragg and his son W. L. Bragg established X-ray crystallography.
Crystals have unique characteristics such as orderly atomic structure, definite external geometric shape, and varying optical and physical properties. Crystal growth can be influenced by factors such as parent liquid supply, temperature variations, and solution strength. Steno’s Law* see below, encapsulates the consistency of angles between corresponding faces in crystals of the same substance. Even in crystals with imperfect growth, this law applies.
Crystal properties depend on their atomic structure and chemical composition. For instance, diamond and graphite, though chemically identical, differ in appearance and properties due to their distinct atomic arrangements. This is known as polymorphism* see below. Certain properties like cleavage and some optical characteristics also vary with the crystal's direction. For example, the colour of a mineral crystal can change depending on the angle of light passing through it.
Crystals form in many shapes and sizes and these habits (shapes) are part of the identification process.
There are 7 crystal systems:
1. Cubic or Isometric, examples: diamond, garnet and spinel.
2. Tetragonal, examples: zircon and scapolite.
3. Hexagonal, examples: beryl and apatite.
4. Trigonal, quartz, tourmaline and corundum.
5. Orthorhombic, examples: topaz and peridot.
6. Monoclinic, examples: moonstone,
7. Triclinic, examples: amazonite and turquoise.
As Amethyst is the gemstone of the month, we will focus on it……
Amethyst is the purple coloured variety of the Quartz (SiO2) family of gem crystals. Its crystal system is trigonal, which usually consists of a hexagonal shape with prisms on each end. It has a hardness of 7 on the Mohs scale which makes it ideal for jewellery. Most gemstones will show colour banding or patches (zebra stripes) which is a diagnostic feature. Most crystals are usually well formed.
Amethyst’s purple colour can range from a light lilac to a deep, intense royal purple, and from brownish to vivid.
Amethyst crystals can be huge. The GIA museum displays a crystal weighing over 74 kgs. (164lbs)
*Steno’s Law (Law of Constancy of Interfacial Angles) “In all crystals of the same substance, the angles between corresponding faces have the same value (when measured at the same temperature)”.
*Polymorphism The ability of minerals having the same composition to crystallise in different crystal systems. "Dimorphism" is used where two species exist e.g. carbon in the form of graphite (hexagonal) and diamond (cubic).