Topaz is a silicate mineral that is crystalline in nature. It has the chemical formula Al 2 SiO 4 (F, OH) 2 and is a rare mineral due to the "F," fluorine. Topaz is an excellent choice for jewelry in any style, with a hardness of 8 and a range of hues, and may even make for a distinctive center stone in an engagement ring. Topaz is named after Thetis, the daughter of Achilles and wife of Peleus. She was chosen by Zeus as the mother of Achilles' son Neoptolemus who would later rule over Mysia. Elemental topaz contains equal amounts of aluminum and silicon and is usually found in combination with other elements such as iron, oxygen, sodium, calcium, potassium, magnesium, titanium, carbon, and phosphorus. It can be blue, green, brown, black, or yellow in color. The brightest topazes are generally blue or grayish-blue, but red, pink, orange, or yellow crystals do occur. In addition to being used in jewelry, topaz is also employed in decorative art, vases, plates, windows, glassware, and optics. In 1729, another reference noted that topaz was "much prized" by jewelers.