Pietersite is the trade name for a (usually) dark blue-gray breccia aggregate made up mainly of hawk's eye and tiger's eye. It was discovered in Namibia by a man named Sid Pieters in 1962. He registered his find in Britain and the discovery was published in 1964, and named pietersite in his honor.
For many years it was believed that this material was only found in Namibia. But in 1993, a similar material was discovered in Hunan Province of China, though specimens did not come to market until 1997. It is now believed that the mine in China has closed, and limited production from Namibia has made pietersite quite difficult to find.
Pietersite belongs to a branch of the tiger's eye family called riebeckite. Tiger's eye is what geologists refer to as a pseudomorph, one mineral that changes into another. Tiger's eye began its life as the mineral crocidolite, a form of asbestos. As quartz replaced the crocidolite it took on the shape of the fibrous mineral and that is what causes the chatoyancy in gemstones of this family.
Unlike tiger's eye, the surface of a pietersite gem looks rather chaotic, with streaks and colors in every direction. This is because during formation of the crystal, the materials that it is composed of were broken apart, swirled every which way, and then were reformed and cemented together by quartz. Stones and crystals that go through that process are referred to as brecciated.
Pietersite colors include blues, rusty reds, golds and browns. The blues range from a baby blue to a dark midnight hue. The Chinese pietersite often occurs in shades of gold, red and blue color segments, which sometimes also include a deep golden brown.
Pietersite is most commonly and almost exclusively cut into cabochons. This is because a tall, round cut is required to maximize the chatoyancy effect. The orientation of the cut is critical with pietersite, since it must be cut exactly parallel to the length of the fibers.