Rhodochrosite, found as rhombohedral crystals of the trigonal system, and in granular masses, usually has a paler and more delicate rose-red colour than rhodonite. Rhodochrosite, the manganese carbonate (MnCO3), as used as an ornamental stone, is characterised by the lovely pink shade of colour, which is variegated and in bands of different shades of pink. The material may be likened in its banding, but not in colour, with green malachite. Indeed, like malachite, the banding is due to stalagmitic formation, the prismatic crystals tending to a radial formation. Grey, fawn arid brown colours are also found.
Although rhodochrosite is found in many different localities, only a few supply the compact material which is used as an ornamental stone, and its use as such began just before the commencement of World War II, when a quantity of this stalagmitic rose-coloured stone came from San Luis in Argentina. The deposit from which it came was said to have been found in a long-disused mine situated on a mountain fissure at a high altitude, which was reputedly worked by the Incas for silver and copper during the thirteenth century. It was owing to this connexion that the names 'Rosinca' or 'Inca rose' were given to the stone. Colorado, Montana and other States of the North American continent are other sources of the mineral. Further localities are Romania Hungary, the Central Provinces of India and Freiberg in Saxony, but these are unimportant as commercial sources for ornamental material.
An important new source of rhodochrosite in both massive and the rarer clear crystal form was discovered in 1974 in a mine at N'chwaning in the Kalahari Desert region of Cape province, South Africa The mineral occurs in a variety of shades in isolated pockets up to 4x2 feet in size, together with other manganese minerals, braunite and bixbyite. Clear crystals are rare, but stones up to 60 carats in weight have been cut and sold for as much as 50 dollars per carat.
The hardness of rhodochrosite is near to 4 on Mohs's scale and it is more easily scratched than is rhodonite. The density is rather variable and lies between 3.45 and 3.70, but the ornamental material has the smaller range of 35-31.65. The refractive indices are 1.820 for the ordinary ray and 1.600 for the extraordinary ray. The negative uniaxial double refraction of 0.220 is large in amount and characteristic of a carbonate mineral. These refractive indices can only be seen on stones cut from clear, nearly transparent, material (crystals), for in the ornamental rhodochrosite the crystalline aggregate precludes anything more than a diffuse shadow edge being seen on the refractometer. The absorption spectrum of the mineral shows a band at 5510A (and other vaguer lines) but the spectrum scarcely assists identification. A small clear crystal showed a band at 4150 A, which could not be seen in the more opaque ornamental types. Under the stimulation of ultra-violet light a dull red glow was observed in samples of the mineral from Argentina and Colorado. The mineral effervesces with acid, but the test is not conclusive for rhodonite may contain some carbonate. A few small stones have been cut from the clearer crystals.
from "Gems, Their Sources, Descriptions and Identification," by Robert Webster and BW Anderson, 1962. Copyright © 1983.
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