Saturday, 10 August 2013

Rhodochrosite Gemstone

Rhodochrosite GemstoneManganese carbonate. Rhodochrosite is a mineral of the calcite series, its name being derived from the Greek rhodon, meaning "pink."

Crystal system

Trigonal

Appearance

It occurs as semitransparent, rhombohedral crystals with poor luster, or as concretionary masses, sometimes with irregular, contorted veining. Normally a definite pink it can be faded pink or slightly orange. Chemical alteration turns it blackish or dull brownish.

Physical properties

Rhodochrosite jewelleryIt has a low hardness of about 4. The density is usually 3.4-3.7 g/cm3. The refractive in-dices are no 1.60, no 1.82. It has perfect rhombohedral cleavage. Genesis Rhodochrosite is found in hydrothermal veins, but also in sedimentary deposits of chemical origin. Occurrence It is quite a common mineral, especially In the United States, Argentina, Mexico, Namibia, Spain, Romania, and the Soviet Union.

Rhodochrosite

Despite its rather low hardness, this mineral is used as an ornamental material because of its pleasing color.

Appearance

The massive material is characterized by a bright pink color, generally in distinct bands (as with agate and malachite), which may be curved or finely contorted. with narrow pale pink or whitish veining. Small slabs are used for mosaics, boxes, pots, and figurines, but the mineral is somewhat brittle. Ills also made into necklace beads or other types of jewelry and recently, quite large, almost transparent crystals have yielded attractive, curved gems.

Distinctive features

Rhodochrosite beads
It has a color similar to that of rhodonite, but is distinguished from the latter by its concretionary structure and by the fact that it is visibly attacked by hydrochloric acid. It is also much less hard. Occurrence The ornamental material mainly comes from Argentina, the United States (Colorado and Montana), Namibia, and the Soviet Union.

Value

Somewhat low. Necklaces of transparent pieces are worth more, although the gem's relative lack of hardness can make it lose its polish easily.


Simulants and synthetics

It is neither imitated nor produced synthetically.

Introduction

Ornamental materials produced by biological processes, whether animal or vegetable, come under the heading of "organic" gems. There are four main organic materials normally regarded as distinct from precious stones, chiefly because of their low hardness, which is a fundamental property of gem-stones. They deserve, nonetheless, to be considered along-side the more prestigious "precious stones," on account of their very pleasing appearance, which makes them invaluable for the preparation of items of personal adornment such as rings, bracelets, necklaces, cameos, and to an even greater extent, decorative objects such as vases and figurines.

Rhodochrosite necklaceThe materials in question are pearl, coral, ivory, and amber, which are all of biological origin, but differ widely in their appearance and chemical composition. The only truly organic substance among them is amber, which is noncrystalline, fossilized resin from conifers. The other three materials consist mainly of inorganic compounds: oxyapatite, or calcium phosphate for ivory and calcium carbonate for the others. This appears in the form of aragonite for pearls and calcite for coral. But none of these chemical compounds is in the pure state, the exact chemical composition varying between specimens, ac-cording to their place of origin color, Hrid age.

This variability in chemical composition produces strong variations in physical properties as well, as may be seen from the following table. The table does not give refractive indices for nontransparent materials, such as pearls, coral, and ivory, but the values of their respective principal mineralogical components, aragonite, calcite, and oxyapatite, respectively.

Note that ivory has much lower density and hardness than its mineralogical components in lire pure stale. This is because it has a relatively low proportion of the chief constituent oxyapetite.

Writer – Curzio Cipriani & Alessandro Borelli

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