Transparent mimetite crystals are extremely rare, and very few have ever been faceted. Cabochons with rich orange and yellow colors have been cut, but this material is too soft to wear as jewelry.
Mimetite belongs to the apatite mineral group and occurs in many locations around the world. However, to date, only Namibia and China have produced very limited quantities of facetable material. With a hardness range of just 3.5 to 4, any cut mimetites are usually reserved for display in gem collections.
With over-the-limit (OTL) refractive indices of 2.128 and 2.147, a very high specific gravity (SG) of 7.24, and piezoelectric properties, mimetite can usually be readily distinguished from more commonly encountered gemstones. However, it shares some of these characteristics as well as color ranges with a few other rarely faceted gemstones: cassiterite, vanadinite, and wulfenite.
Cassiterite and vanadinite aren’t piezoelectric and typically have lower SG than mimetite. However, mimetites may have lower SG values if calcium (Ca) replaces lead (Pb) in its composition. Vanadinites usually have higher refractive indices (RIs) than mimetites, while cassiterites usually have lower RIs.
Wulfenites are well-known for their rich, red colors, but some may have yellow, orange, or brown colors. They also have piezoelectric properties and SG values close to mimetites but typically higher RIs.
All these gems are rare, but you’re more likely to encounter faceted cassiterites and wulfenites than faceted mimetites and less likely to encounter faceted vanadinites. They all have distinctive hardness values, but conducting scratch tests on finished gems, especially ones so rare, isn’t recommended.
Mimetites also have colors and properties that overlap or approximate those of pyromorphites. Distinguishing them may require advanced chemical analysis.
Mimetite crystals, Hat Yai District, Songkhia Province, Thailand. Photo by Géry Parent. Public Domain.
Stone Sizes
Gem cutters have made unusual, interesting cabochons up to an inch or two in length from globular Mexican mimetites.
Most transparent crystals, particularly from Tsumeb, are extremely rare and are usually preserved as mineral specimens rather than cut. However, small broken crystals have been cut, yielding some stones up to a few carats in weight, to a maximum of 5-7 carats.
Care
You’re more likely to find mimetites, if at all, in mineral collections than jewelry collections. Their low hardness makes them very susceptible to scratching, so reserve any mimetite jewelry for occasional wear and use protective settings.
Mimetites contain arsenic and lead, both highly toxic. While this shouldn’t pose a problem for wear or display, faceters should take precautions against inhaling or ingesting particles, protect their eyes, wash their hands, and clean their workspace after cutting mimetites. They should wear protective masks and, ideally, use a glovebox during cutting, polishing, and cleaning.