- Corundum is better known by two of its more valuable forms, rubies and sapphires. The stone's color depends on impurities within the corundum. But corundum appears in many other colors besides red and blue. The stone is cut in a variety of shapes, including mixed cut, brilliant cut, step cut and cabochon. When a corundum is asteriated (having an asterism), the stone is generally cut into a cabochon, then polished so that the rutile inclusion (causing the asterism) produces a six-ray star effect that generally appears as white over the top of the gemstone.
- Opals are a hardened silica gel and usually contain 5 to 10 percent of water. Therefore, if an opal dries out, it can crack. While opals can be faceted, they are usually cabochons and beads. Common (also called potch) opals are not iridescent. Precious opals possess flashes of iridescence caused by the stone's arrangement of tiny silica spheres. Opal is found in cavities in sedimentary rocks or in igneous rock veins. It can replace organic material in fossil wood, shell and bone.
- Jade is most often carved into figurines or cut into beads and rectangular shapes. There are two varieties of jade: jadeite and nephrite. Jadeite can be green, lilac, white, pink, brown, red, blue, black, orange and yellow. But the imperial jade, an emerald green color, is the most highly prized. Nephrite jade ranges from dark green to cream-colored and may be homogeneous, blotchy or banded. When polished, jadeite commonly has a dimpled surface.
- The most popular beryls are emeralds and aquamarines. Most emeralds from historical jewelry came from Cleopatra's mines, which now yield only poor quality emeralds. Emeralds are rarely flawless. To hide flaws and enhance color, the stones are often oiled. The sea green aquamarine was the highest prized in the 19th century. Today, sky blue and dark blue are the most valued. Aquamarines are dichroic, meaning the stone may appear blue or colorless depending on the viewing angle. To emphasize the deepest coloration, an aquamarine is cut with the table facet parallel to the crystal's length. Pure, colorless beryls are goshenites, which have been used to imitate diamonds and emeralds by placing either a silver or green piece of metal foil behind the stone.
- There are many varieties of garnets. Regardless of the variety, garnets have no cleavage. Pyrope garnets are blood red and come from volcanic rock and alluvial deposits. In Bohemia, pyrope jewelry has been fashioned for more than five centuries. The darker red (sometimes nearly black) garnet is almandine. Almandines may produce a four-ray star effect when cut as a cabochon. Because the stone can be so dark and is often opaque or subtranslucent, the underside may be hollowed out to increase the amount of light filtering through.
