Gemstones silicate

Mineral gemstones that have the same basic chemical composition, that is, are composed of the same major elements and differ only in color, are considered as variations of the same mineral species. As gemstones, however, minerals that have the same composition and crystalline structure but exhibit different colors are classified as different gemstones. Beryl, for example, a mineral (composed of beryllium aluminum silicate), includes a pink variety, known by the gemstone name of morganite, and also a well-known green variety, emerald. Table 18 lists and classifies, by composition and color, gemstones that have been appreciated since antiquity. 

The ability to geographically identify the source area of natural gems plays an important role in determining if these stones may have originated from a politically unstable area. Garnet is a semiprecious silicate mineral of variable composition that has been used as a gemstone since the Bronze Age. 

Zirconium silicate (ZrSiO ) is one form of the mineral whose crystals when polished are known as cubic zircons, which resemble diamond gemstones. 

Aquamarine, like emerald, is a transparent variety of beryl, or beryllium silicate. Its light blue to blue-green color results from small amounts of iron in the crystal. Like most beryl stones, it measures 7.5 to 8 on the Mohs hardness scale. Most aquamarine gemstones come from Brazil. 

Tourmaline is a highly complicated silicate, with a wide range of compositions and colors. It probably exhibits more colors than any other kind of gemstone. Sometimes there are several different colors in the same crystal. Watermelon tourmaline, for example, is green on the outside but red in the middle. 

Lapis lazuli is a deep blue gemstone that is a complex copper silicate mineral varying widely in composition. It often contains sparkles of iron pyrite or calcite. The best source is probably Afghanistan. A pale blue variety is found in Chile. Some material sold as lapis lazuli is actually artificially colored jasper from Germany. 

In 1985 benitoite became the California state gemstone. Found only in a tiny mine near Coalinga, California, it is a silicate of harium and titanium with trace impurities that cause a range of hues from colorless to blue to pink. Its general formula is BaTi(Si03)3. If a 15-carat stone were pure BaTi(Si03)3, how many moles of silicon would it contain ... 

One of the most well-known silicate minerals is quartz (Si02), which consists of a continuous three-dimensional network of silica and oxygen without any atomic substitutions. It is the second most abundant continental mineral, feldspars being most abundant. The network of covalent bonds (between silicon and oxygen) is responsible for the well-known hardness of quartz and its resistance to weathering. Although pure quartz is clear and without color, the presence of small amounts of impurities may result in the formation of gemstones such as amethyst. 

Garnet crystals are one of the older popular gemstones and a special class of mineral (much like spinel). The crystal structure is able to accommodate many different cations, which then produce different colors. All those shown in the box (on the next page) are natural silicates, although several synthetic garnets are now available synthetic garnets produced for gemstones are usually doped YAG. 

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