Magnesite

Magnesite, also known as “Wild Horse Turquoise,” not having copper nor iron that give turquoise their natural blue and green colors, respectively, is not a true turquoise. Magnesite, rather, is a type of calcite. Specifically, it’s magnesium carbonate (MgCO3) and commonly includes variable quantities of iron and calcium carbonates (FeCO3 and MgCO3, respectively). These latter carbonates have crystalline form similar to magnesium carbonate, and, in combination with the magnesium carbonate, form uniform, solid mineral solutions, regardless of the proportions of the metallic components.

Africa, China, Korea, Brazil, Europe, and the United States mine Magnesite, and the Globe Copper Mine, in Arizona, produces magnesite that’s very popular in jewelry because it’s encrusted in an aesthetically appealing matrix of hematite. Rich in magnesium, magnesite is valued for the production of Epsom salts and fireworks.

Magnesite forms when magnesium-rich rocks, like serpentine and dolomite, come into contact with water rich in carbon dioxide (CO2). Precipitation or metasomatism (change from one mineral form to another), of the minerals from the slightly acidic water, forms trigonal crystals that cleave off into rhombohedrons, or boxy-looking gems … very similar to other calcite gems.

Natural magnesite most frequently forms opaque, white, microcrystalline, porous stones with a dull luster, often described as looking like un-glazed porcelain. It looks similar to howlite, dolomite, and marble and may be confused with these minerals. Magnesite masses may occur within light or dark host rock matrix, giving it a webbed, mottled, or streaked appearance.

Aside from the typical white magnesite, sources report colors like gray, yellow, brown, tan, orange, and light pink, and some material may fluoresce in ultraviolet light. In fact, magnesite miners, in Nevada caves, use ultraviolet lamps to find the gems.