Dunite:
A rock made entirely of olivine.

Harzburgite:
A rock that contains more olivine than pyroxene.

Peridote:
Gem-quality olivine is called peridote.

Peridotite:
Peridotite has two varieties -- harzburgite (a rock that contains more olivine than pyroxene) and dunite (a rock consisting almost entirely of olivine). Peridotite typically displays a coarse crystalline texture and is both very dark and very dense. In harzburgite, the pyroxene is greenish, grayish to black. Some peridotite is rich in bronzite, a golden-brown, massive crystalline iron-rich variety of the mineral enstatite. In harzburgite and dunite, the olivine is brown, greenish brown to black and has a more glassy texture than the pyroxene. Accessory minerals include magnetite, chromite, and serpentine minerals.It may contain traces of plagioclase.

Furnaces and Kilns
Furnaces used to bake bricks are lined with olivine. Olivine is used as a refactory liner in kilns and heating furnaces.

Gems and Jewelry
Peridote is the best known gem variety of olivine, a species name for a series of magnesium-iron rich silicate minerals. This bright yellow-green to green gemstone has caught the fancy of humans for thousands of years. Some historians even suspect that at least some of the "emeralds" worn by Cleopatra were actually peridot. Much of its recent popularity can be explained by its currently being recognized as the birthstone for the month of August, people wear the stone because it is supposed to bring the wearer success, peace, and good luck. Currently, the United States is the basic suppler of peridote to the world gemstone industry. Deposits in Arizona are the major source of U.S. peridot. Faceted Arizona peridote is highly prized locally, but also enjoys widespread popularity for the manufacture of birthstone and other jewelry. The bright green, yellow-green to olive-green peridote is used both as a faceted and tumbled gem. The faceted stones are used in rings, earrings, pendants, bracelets, and as accent stones. The tumbled gems can be set in baroque jewelry, drilled and strung as beads, used in mosaics, and in the manufacture of gem trees.



-- Excerpts from:
USGS/NPS Geology in the Parks Website, 2002; Gemstones, An Overview of Production of Specific U.S. Gemstones: U.S. Bureau of Mines Special Publication 14-95; and North Carolina Geological Survey Website, 2002