Of all the minerals used as gems, non compares with quartz in its diversity of occurrence and in abundance of varieties. It is found in igneous, sedimentary and metamorhic rocks and in some is essentially the only mineral present. Because of its hardness, general lack of cleavage and chemical stability it survives and maintains its identity as pebbles of stream beds and sand grains of sea beaches.
Quartz as the fine-grained variety, flint was the mineral used by man. From it were fashioned tools and weapons that started him on the long road to civilization. Later, it was undoubtedly colored varieties of quartz that were first used as ''gems'' for both personal adornment and for their presumed magical properties. The earliest work on minerals that has survived was written by Theophrastus about 300 B.C. In this treatise, On Stones, quartz and its varieties are mentioned more often than any other mineral. And there is the observation that among the ancients there was no precious stone in more common use than sardion, a red chalcedony.
This stone, called Chalcedony, is a variety of silicate (SIO2) of microcrystanlline texture. Its largest bed (circa 5 million tons of reserve) in the world is located of 50 km.north of Eskisehir, Central Anatolia. Eskisehir Chalcedonies range from light to dark blues, sometimes purple and exist in mixtures with opals and agates and even in narrow blue and white striped species (Lace Chalcedony).
Chalcedony, abundantly used in decorative and jewellery industries of the world, is supplied from different countries. Quartz as a gem mineral is divided in the following discussion into two groups: coarsely crystalline varieties, which many gemologists refer to as quartz, and two fine grained, microcrystalline to crypocrystalline varieties, which although physically different are often grouped in gemology under the species name Chalcedony.
Microcrystalline Varieties of SIO2 Depending on the nature of their microcrystalline to cryptocrystalline units, the fine-grained varieties of quartz can be divided into types, fibrous and granular. Because of their fine textures, it is frequently difficult to distinguish beween them without examination of small particles with a high-power microscope. In practice, femologists do not make the distinction and refer to all such materialsas chalcedony treating it in effect as a gem species with numerous varieties.
Fibrous Varieties. The different varieties of chalcedony are given special names based on degree of trensparency, color, color distribution and phenomena. Specifically, the name chalcedony is used a lone to designate a light honey-yellow to gray, translucent material with a wawy luster.
It commonly accurs in botryoidal masses with the fiber length at right angles to the hummocky surface. Chacedony has a lower SG (2.60) than coarsely crystalline quartz because of a slight porosity. The porous nature of the mineral permits it to be colored by treating it with chemical solutions: black brown, sugar and sulfuric acid; ref, iron oxide; blue, potassium ferrocyanide and iron sulfate; yellow-green, hydrochloric acid; deep green, chromic acid. Most chalcedony, usually cut en cabochon, has been color enhanced.