Aragonite & Calcite

Posted on July 1, 2022 by John J. Bradshaw, GIA GG

INTRODUCTION Just as graphite and diamond have the same chemical composition but crystallize in two different systems, so is the case with aragonite and calcite. Both have the formula CaCO3, aragonite in the orthorhombic system and calcite in the trigonal system. Both are strictly in the realm of the gemstone collector as neither has the hardness or durability for use in jewelry. Although there are other similarities, there are large differences as well, just as there are between graphite and diamond. Aragonite, 8.61 ct from Bilin, Czech Republic. Faceted and photo by John Bradshaw, LOCALITIES Gem quality aragonite is almost exclusively from Bilin, Czech Republic. Calcite is a common mineral in the earth’s crust. Most are unaware that marble is the granular aggregate variety of calcite. Gem quality calcite, suitable for faceting, is mostly found in Brazil, Mexico, the United States (New York), Canada, Germany, Kazakhstan, and Iceland. Calcite from Ice- land is nicknamed “Iceland spar” and is likely the source for the Vikings “sunstone.” Researchers have determined that Vikings used transparent calcite crystals to fix the bearing of the sun while navigating the ocean on cloudy days when the sun was obscured. COLORS Aragonite has a relatively small range of appearance from colorless to yellow with saturation ranging from grayish to brownish (Figure 1). Calcite on the other hand, occurs in a variety of colors. It can be and is often colorless. On rare occasions, two transparent colorless crystals grow in a confined space causing a twinning plane between the two crystals. If this twinning plane is properly oriented in the faceted stone, it creates an optical effect commonly called “rainbow” calcite (Figure 2). Yellow calcite is probably the next most common color (Figure 3). It can also occur in orange and brown colors. Pink calcite, also known as cobaltocalcite, is due to impurities of cobalt (Figure 4). CLARITY Both aragonite and calcite should be eye clean with high transparency. Cobaltocalcite is semitransparent at best, sometimes with small inclusions.   Rainbow calcite, 52 ct from Balmat, New York. Faceted by Art Grant, Photo by Tino Hammid. STONE SIZES/CUTTING There is a large difference in stone sizes of aragonite vs. calcite. Faceted aragonites are usually less than 15 ct with a few up to 50 ct. The largest on record is a 110 ct emerald cut in a private collection. Calcite gems are usually not cut less than 50 ct un- less of an unusual locality or rare color. Cobaltocalcite has been cut up to 20 ct but most tend to be in the 1-5 ct range. One of the reasons for the large discrepancy in sizes of arag- onite and calcite is the degree of difficulty in cutting. Most carbonates (rhodochrosite, magnesite, dolomite, siderite...

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