Rock Crystal

Rock crystal can form in any geological environment where quartz forms; large, well-formed transparent crystals come from some pegmatites, Alpine-type fissures and hydrothermal veins. It is found worldwide, but few deposits are worked commercially: the Ouachita Mountains of Arkansas and the Herkimer area of New York (USA), Minas Gerais (Brazil, currently the most important exporter), the Polar Urals (Russia), Madagascar and Nepal are among those named by The Quartz Page. Gem references also list the Swiss Alps, Japan, New South Wales (Australia), Myanmar and Canada. Arkansas is the largest US producer of gem and decorative rock crystal.

Rock crystal is identified as quartz (refractive index 1.544–1.553, birefringence 0.009, specific gravity about 2.65, hardness 7) that is colourless and transparent; it may contain many kinds of inclusions, such as two-phase fluid inclusions, rutile needles, tourmaline, actinolite or chlorite. It is used to imitate diamond, and doubly terminated crystals from Herkimer, New York, are misleadingly called "Herkimer diamonds". Treated material exists: quench-crackled and dyed stones show a web-like network of cracks, iridescent "iris" or "rainbow" quartz owes its colours to cracks, and coated stones carry an ultra-thin metallic layer (such as gold for the light-blue "Aqua Aura"). Many rock crystals can be turned into smoky quartz by irradiation, and most smoky quartz from Arkansas is irradiated rock crystal. Quartz triplets are common imitations, and synthetic rock crystal is used only industrially.

With a hardness of 7 and no real cleavage, quartz is rated as having very good wearability and needs no special care instructions. It is nevertheless brittle, with a conchoidal fracture, so a hard knock can chip it. Quench-crackling weakens a stone, and some dyes can fade. Broken crystals can have razor-sharp edges, and quartz should not be ground dry because long-term exposure to fine dust can cause silicosis.

The word "crystal" goes back to the Greek krystallos, recorded by Theophrastus around 300 BCE; the ancients believed rock crystal was ice frozen so deeply that it could not melt again. Rock crystal has been gathered since Palaeolithic times, tools and weapons were made from it before it was used for ornament, and records show its use in decoration and jewellery for at least 4,000 years. It was made into rings, bracelets, seals, cups and carvings; in ancient Egypt it formed the cornea of inlaid stone eyes in statues. In the late 1980s and early 1990s demand from the metaphysical market accounted for 40% or more of US rock-crystal consumption, later settling at about 15–20%.

Japanese crystal retailers say that with power stones one "begins with crystal and ends with crystal". The retailer Pascle writes that crystal is said to stand for good fortune, purification, warding off evil and the fulfilment of wishes, and that it is used as an all-purpose charm; it also notes that rock crystal is sold in Japan as an April birthstone.