A cryptocrystalline mineral of mountain-green, bluish-green, sky-blue and turquoise-blue colour often with an opal or enamel-like texture, chrysocolla is a hydrous copper silicate (CuSiO32H2O), but the composition is somewhat variable. Indeed, through impurities such as free silica, alumina, black oxide of copper, limonitic oxide of iron and manganese oxide the colour can vary from those given above to brown and black. Much chrysocolla may be a mineral gel with copper oxide, silica, and water in varying proportions depending upon the conditions of formation. Thus, the constants of the mineral are variable. The mean refractive index is 1.50 (to = 1.46 and e = 1.57 have been determined in the case of microscopic acicular crystals from Mackay, Idaho). The hardness varies from 2 to 4 on Mohs's scale, rising to over 6 if much quartz be present, and the density varies between 2.00 and 2.45.
Chrysocolla is a mineral of secondary origin occurring in the oxidised zones of copper veins at widespread localities, particularly at Nevada, New Mexico and Arizona in the United States of America. Other localities are at Katanga in the Congo, in Russia and Chile. Much of the chrysocolla of jewellery consists of very attractive cabochons of green or blue chrysocolla impregnating either rock crystal or opal. Such material will have the hardness (7) and the refractive indices of quartz; the density may be slightly lower than the 2.65 for rock crystal owing to the slight lowering by the included chrysocolla.
It may be convenient to include in this section the stone called 'Eilat Stone', or 'Elath Stone', which is found near Eilat at the heart of the Gulf of Aqaba on the Red Sea. It is said to come from the copper mines of King Solomon. The rock, which is usually cut into cabochons or 'tumbled', is a mixture of copper minerals. The constituents vary considerably, but include chrysocolla, turquoise and pseudomalachite, a copper phosphate mineral which accords to the formula Cu3P2O83Cu(OH) and resembles malachite in colour. Other copper minerals may be included, and in one case apatite was determined. The colour, which is variegated, is blue to green. The density, on specimens determined, varied from 2.8 to 3.2, and like most copper minerals the stone turns yellow when a spot of hydrochloric acid is applied.
from "Gems, Their Sources, Descriptions and Identification," by Robert Webster and BW Anderson, 1962. Copyright © 1983.
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