Quick answer: Pietersite is a brecciated tiger’s eye: fibrous quartz and amphibole that was shattered by geological forces and recemented, so its shimmering fibers swirl in many directions instead of running parallel. It was discovered in Namibia by Sid Pieters in 1962, and later deposits were found in China.
Tiger’s eye shows one tidy band of light that rolls across the stone as you tilt it. Pietersite shows the same light, but everywhere at once and in every direction: swirls of gold, blue, and dark grey that shift like weather. The reason is that pietersite is tiger’s eye that was shattered by geological forces and then healed back together.
A recent name
Pietersite is named for Sid Pieters, a Namibian gem dealer who described material he found in Namibia in the early 1960s. Similar material was later reported from China. Because the classic Namibian deposit is small and has produced intermittently, good pietersite has always been scarce compared with ordinary tiger’s eye.
How tiger’s eye forms first
Tiger’s eye and its blue-grey cousin hawk’s eye form in banded iron-rich rocks that contain a fibrous blue amphibole called crocidolite, a variety of riebeckite. Quartz grows in thin veins alongside and around those fibers, repeatedly cracking and resealing, so the finished stone is quartz packed with parallel fibers. Where the iron in the fibers has altered to iron oxides the color turns golden brown, and where it has not the stone stays blue-grey.
The parallel fibers are what make the band of light. Light reflects off every fiber at the same angle, so it gathers into a single bright line perpendicular to them. That effect is called chatoyancy, and we explain the physics in chatoyancy and asterism.
Then it breaks
In pietersite the fibrous rock was later deformed: folded, crushed, and broken into angular fragments. Geologists call rock made of broken angular pieces a breccia. Fresh silica-rich fluid then flowed through and cemented the fragments back together. Each fragment still has its fibers, but now they point in random directions. The result is dozens of small chatoyant zones, each catching the light at a different angle. Turn the stone and different patches light up while others go dark, which is why the material is sometimes nicknamed “tempest stone.”
Why it is almost always cut as a cabochon
Chatoyancy only shows well on a curved, polished surface, and it shows best when the fibers run parallel to the base of the stone. With pietersite the fibers go every way, so the cutter’s job is to find the orientation where the most zones flash at once. That is careful, material-wasting work, and it is one reason good pietersite cabochons carry a premium. Our post on how a cabochon is cut walks through the process.
What you are actually holding
- Mineral: mostly quartz, with the original amphibole fibers replaced or encased in silica.
- Hardness: about 7, like other quartz, so it wears well in jewelry.
- Handling: the fibers are sealed inside quartz, so a finished, polished stone is simply handled like any quartz gem. Lapidaries cut it wet, as they do with all silica materials, to keep dust down.
- Treatment: natural pietersite is not usually dyed, but some tiger’s eye on the market is dyed or heated to change the color. Our guide to dyed, heated, and coated stones covers what to look for.
There is a neat piece of geology in every piece: the stone formed once, was destroyed, and formed again. The pseudomorphs post covers other minerals that carry evidence of an earlier life inside them.
