Turquoise is an opaque, blue-to-green mineral that is a hydrous phosphate of copper and aluminum.  It is rare and valuable in finer grades and has been prized as a gem and ornamental stone for thousands of years owing to its unique hue.  In recent times turquoise, like most other opaque gems, has been devalued by the introduction of treatments, imitations, and synthetics onto the market, some difficult to detect even by experts.
The substance has been known by many names, but the word turquoise was derived around 16th century from the French language either from the word for Turkish (Turquois) or dark-blue stone (pierre turquin).  This may have arisen from a misconception: turquoise does not occur in Turkey but was traded at Turkish bazaars to Venetian merchants who brought it to Europe.  The color, however, has been employed extensively in the decorative tiles adorning Turkish places of worship and homes for hundreds of years, beginning with the Seljuks, and the association quite possibly has caused the name to take root.

Turquoise was among the first gems to be mined, and while many historic sites have been depleted, some are still worked to this day. These are all small-scale, often seasonal operations, owing to the limited scope and remoteness of the deposits. Most are worked by hand with little or no mechanization. However, turquoise is often recovered as a byproduct of large-scale copper mining operations, especially in the United States.

The Southwest United States is a significant source of turquoise; Arizona, California and Nevada are (or were) especially rich. The deposits of California and New Mexico were mined by pre-Columbian Native Americans using stone tools, some local and some from as far away as central Mexico. Cerrillos, New Mexico is thought to be the location of the oldest mines; prior to the 1920s, the state was the country's largest producer; it is more or less exhausted today. Only one mine in California, located at Apache Canyon, operates at a commercial capacity today.

Untreated turquoise, Nevada USA. Rough nuggets from the McGuinness Mine, Austin; Blue and green cabochons showing spiderweb, Bunker Hill Mine, Royston

Nevada is the country's other major producer, with more than 120 mines which have yielded significant quantities of turquoise. Unlike elsewhere in the US, most Nevada mines have been worked primarily for their gem turquoise and very little has been recovered as a byproduct of other mining operations. Nevada turquoise is found as nuggets, fracture fillings and in breccias as the cement filling interstices between fragments. Because of the geology of the Nevada deposits, a majority of the material produced is hard and dense, being of sufficient quality that no treatment or enhancement is required. While nearly every county in the state has yielded some turquoise, the chief producers are in Lander and Esmeralda Counties. Most of the turquoise deposits in Nevada occur along a wide belt of tectonic activity that coincides with the state's zone of thrust faulting. It strikes about N15E and extends from the northern part of Elko County, southward down to the California border southwest of Tonopah. Nevada has produced a wide diversity of colors and mixes of different matrix patterns, with turquoise from Nevada coming in various shades of blue, blue-green, and green. Nevada produces some unique shades of bright mint to apple to neon yellow green. Some of this unusually colored turquoise may contain significant zinc and iron, which is the cause of the beautiful bright green to yellow-green shades. Some of the green to green yellow shades may actually be Variscite or Faustite, which are secondary phosphate minerals similar in appearance to turquoise. A significant portion of the Nevada material is also noted for its often attractive brown or black limonite veining, producing what is called "spider web matrix". While a number of the Nevada deposits were first worked by Native Americans, the total Nevada turquoise production since the 1870s has been an estimated at more than 600 tons, including nearly 400 tons from the Carico Lake mine. In spite of increased costs, small scale mining operations continue at a number of turquoise properties in Nevada.

In an attempt to recoup profits and meet demand, some American turquoise is treated or enhanced to a certain degree.  These treatments include innocuous waxing and more controversial procedures, such as dyeing and impregnation. There are however, some American mines which produce materials of high enough quality that no treatment or alterations are required. Any such treatments which have been performed should be disclosed to the buyer on sale of the material.

For at least 2,000 years, the region once known as Persia, has remained the most important source of turquoise, for it is here that fine material is most consistently recovered. This "perfect color" deposit, which is blue naturally, turns green when heated because getting dehydrated is restricted to a mine-riddled in Neyshabur, the 2,012-metre mountain peak of Ali-mersai, which is tens of kilometers from Mashhad, the capital of Khorasan province, Iran. A weathered and broken trachyte is host to the turquoise, which is found both in situ between layers of limonite and sandstone, and amongst the scree at the mountain's base. These workings, together with those of the Sinai Peninsula, are the oldest known.

Iranian turquoise is often found replacing feldspar. Although it is commonly marred by whitish patches, its color and hardness are considered superior to the production of other localities. Iranian turquoise has been mined and traded abroad for centuries, and was probably the source of the first material to reach Europe

Pure topaz is transparent but is usually tinted by impurities; typical topaz is wine or straw-yellow. They may be made white, gray, green, blue, pink or reddish-yellow and transparent or translucent.

Topaz can also be irradiated, turning the stone blue, ranging from a light pure color to very dark almost electric blue.  A recent trend in jewelry is the manufacture of topaz specimens that display iridescent colors, by applying a thin layer of titanium oxide via physical vapor deposition, this stone is then sold as 'mystic topaz'.

Designs by Cherie Thum

Topaz is commonly associated with silicic igneous rocks of the granite and rhyolite type. It typically crystallizes in granite pegmatite's or in vapor cavities in rhyolite lava flows like those at Topaz Mountain in western Utah. It may be found with fluorite and cassiterite. It can be found in the Ural and Ilmen mountains, Afghanistan, Czech Republic, Germany, Norway, Pakistan, Italy, Sweden, Japan, Brazil, Mexico, and the United States.

Topaz crystals from Brazilian pegmatite's are up to 80 cm x 60 cm x 60 cm in size.  The biggest topaz crystal ever found, named "El Dorado", was found in Brazil in 1984. It weighs 6.2 kg and belongs to the British Royal Collection. The famous Braganza diamond is in most likelihood a Topaz. The Topaz of Aurungzebe, observed by Jean Baptiste Tavernier measured 157.75 carats.

The name "topaz" is derived from the Greek Τοπάζιος (Τοpáziοs), which was the ancient name of St. John's Island in the Red Sea which was difficult to find and from which a yellow stone (now believed to be chrysolite: yellowish olivine) was mined in ancient times; topaz itself (rather than topazios) wasn't really known about before the classical era.   In the Middle Ages the name topaz was used to refer to any yellow gemstone, but now the name is only properly applied to the silicate described above.

Many modern English translations of the Bible, including the King James Version mention topaz in Exodus 28:17 in reference to a stone in the Hoshen: "And thou shalt set in it settings of stones, even four rows of stones: the first row shall be a sardius, a topaz, and a carbuncle: this shall be the first row." However, since these translations as topaz all derive from the Septuagint translation tòpazi[òs], which as mentioned above referred to a yellow stone that wasn't topaz, probably chrysolite, it should be borne in mind that topaz is not meant here.  The masoretic text (the Hebrew on which most modern bible translations of the Old Testament are based) has pitdah as the gem the stone is made from; pitdah is of unknown meaning, though scholars think it is related to an Assyrian word meaning flashed. There is a wide range of views among traditional sources about which tribe of the Israelites in which the stone refers.