Semiprecious gemstone

A special class of stones are the precious and semiprecious gemstones, with a long history of use in decorations and jewelry. Especially in the Far East and pre-Columbian America, jade was used extensively for carving ceremonial and luxury items. Jade encompasses two different minerals quite similar... 

In addition to the minerals, there are also some rock-forming homogeneous materials that have neither the definite chemical composition nor the distinctive crystal structure characteristic of minerals. Such materials cannot, therefore, be considered as minerals and are known as mineraloids. Obsidian, for example, a natural material that has been widely used since prehistoric times for making lithic tools and decorative objects, is a mineraloid. Obsidian has neither a definite chemical composition nor a characteristic crystal structure and is not, therefore, a mineral. Copal and amber are other mineraloids that since antiquity have been treasured as semiprecious gemstones. 

Turquoise Semiprecious gemstone Hydrated phosphate of aluminum and copper Carved with relative ease... 

In addition to silica (silicon dioxide SiO ), the crystal form of silicon is found in several semiprecious gemstones, including amethyst, opal, agate, and jasper, as well as quartz of varying colors. A characteristic of quartz is its piezoelectric effect. This effect occurs when the quartz crystal is compressed, producing a weak electrical charge. Just the opposite occurs when electric vibrations are fed to the crystal. These vibrations are then duphcated in the crystal. Quartz crystals are excellent timekeeping devices because of this particular characteristic. 

The semiprecious gemstone tourmaline, with an approximate formula CaLi2Al7(OH)4-(B03)3Si60i8, has a pyroelectric coefficient, 7r of 4 X 10 C m K-. The unique polar axis is the crystallographic c axis. What is the change in polarisation caused by a change of temperature of 100 °C ... 

Precious and semiprecious gemstones are examples of ionic compounds called minerals that are cut and polished to make jewelry. For example, sapphires and rubies are made of aluminum oxide, AI2O3. Impurities of chromium make rubies red, and iron and titanium make sapphires blue. 

---