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(And its fascinating varieties)
Quartz, silicon dioxide, is one of the main components of the Earth's crust with a considerable hardness (7 on the Mohs scale) and quite a low density (2.65 g/cm3).
It has a seemingly hexagonal crystal structure made of trigonal crystallized silica. Quartz crystals are sometimes large and may have very peculiar physical and optical characteristics. They often cluster and form either Geodes, if crystallisation takes place towards the inside of a rock cavity, or Druses if it occurs towards the outside.
Quartz is resistant, durable, and beautiful. It is also a common and therefore affordable mineral. This should not however prevent from appreciating the gemmological significance of quartz, which results from its physical and aesthetical qualities.
There are many types of quartz, some of which are very well known. Among them, the following are of note:
Large individual crystals are usually classified into one of the following main categories according to their colour: ROCK CRYSTAL (colourless), AMETHIST (purple), and the so-called CITRINE TOPAZ (yellow) among others. ROSE QUARTZ is sometimes to be found in large crystals or amorphous materials.
It may contain inclusions of other minerals such as Rutile. The slender golden needles of this mineral which penetrate quartz crystals have a hair-like look, for which reason these are also known as VENUS HAIRSTONES. Quartz may also contain TOURMALINE in the form of elongated, usually black crystals.
These crystals may sometimes have little green mica inclusions, resulting in a stone of an almost homogeneous green colour which is known by the name "INDIAN JADE" (AVENTURINE).
Another equally beautiful category is that of microcrystals. They are organised into a number of layers, thus creating different colour-banded structures: AGATES.
Other chatoyant mineral inclusions may result in fantastic gemstones, such as TIGER’S EYE and MOSS AGATE.
One of the most appreciated agates or chalcedonies is CHRYSOPRASE. Its colour is normally apple-green, but varies to deep green to the point that some transparent specimens may be mistaken for emeralds "the colour of which resembles that of a young budgerigar’s throat, of Khadyota’s back, of sprouting grass, of the froth of the sea and of the ends of a peacock’s tail feathers" (Ratnapariska or "The knowledge of gems" Author unknown, India).
Artists have made the most of its unique and colourful banded structure for shaping. This art of carving stones is known as GLYPTICS. It underwent major development in Roman times, when seals were engraved and later stamped on molten wax by way of "signature". On these seals there used to be a relief, and also a number of pretty images for decoration. This technique is still used today to carve coats of arms on the differently coloured layers of onyxes and agates which may be mounted on rings or the like. This technique has also been used to carve images on the shells of some mollusc which have different colour layers as well. This is the case of the CAMEOS, ever so popular in Italy.
Quartz may also appear in the form of granular aggregates. These make up some beautiful varieties which include JASPERS of different colours with unpredictable spots and veins (Red, Green, Heliotrope, Bloodstone, Sard, Leopard skin, Mokaite, etc).
The amorphous varieties of quartz such as FLINT are also interesting from a gemmological point of view. When high quality flint contains a specific water content, it is called OPAL. Opals are very valuable stones with quite particular plays of colour which have given rise to imaginative legends all through history. Occasionally, quartz has "fossilised" tree trunks into "WOOD OPALS" or may have included animal matter, such as skeletons, radiolarians, or diatom shells.