At auction last year I purchased a flat of mixed
minerals (actually several flats) since surprise finds in the mineral world (such
as auction boxes) bring out the best in my temperament! One of the auction specimens was a black
mineral with streaks/fibers of a lighter colored (ivory to white) mineral. The label stated “Pietersite, South Africa.” At first glance I wondered why anyone would
want a specimen like this, and what in the world was pietersite. So, I embarked on a journey of
Rough cut pietersite (the beast) and a cut and polished cabochon (the beauty). Length of rough cut ~2.9 cm, of cab ~1.9 cm.
Turns out that pietersite (SiO2) is a type or variety of chalcedony (which is actually a variety of quartz) with included fibers of an amphibole mineral, perhaps riebeckite or its fibrous variety, crocidolite, or perhaps even their pseudomorphs. It is similar in appearance to tiger’s eye; however tiger’s eye is not chalcedony but quartz (macrocrystalline variety). Some gem cutters believe pietersite is related to hawk’s eye and the latter is related to tiger’s eye. Hawk’s eye is formed when SiO2 replaces the fibrous crocidolite and creates iridescent planes or fractures with a silky luster. Then tiger’s eye is created when the iron from the decomposed crocidolite oxidizes and changes to a golden brown color. Pietersite is then formed when the fibrous structure of both tiger’s eye and hawk’s eye are brecciated or broken. The brecciated fragments are later cemented together by SiO2.
Almost all pietersite on the market comes from the Outjo District, Kunene Region, in southern Africa, and colors range from golden to brown to black. There are reports of a bluish pietersite coming from China but "rumors” on the internet indicate the mine is shut down.
I guess the end result is that pietersite is not actually a recognized mineral but a trade name for chalcedony with amphibole inclusions. It seems fairly rare, being mined at a single locality in Africa. The cabs are quite nice and very chatoyant. So, take a look at your next visit to the rock and mineral store.