Some minerals tend to form long, thin, closely packed needlelike structures that look like fibers or hair. Sometimes a mineral's threads or fibers are flexible enough to bend. Chrysotile, a magnesium-containing member of the serpentine group of minerals, often grows in long fibers, which inspired scientists to name the substance after the Greek words for "hair of gold." Tremolite and actinolite, both part of a different group of silicate minerals formed within metamorphic rocks, may occur as parallel or radiating fibers. The crocidolite variety of the mineral riebeckite comes in dark blue or gray needles or fibers and is a significant component of the beautiful gemstone known as tiger's eye.