For most of modern times Topaz has been known as a yellow to golden gemstone although in the last few decades brilliant blue topaz has come onto the market, thanks to irradiation of pale or clear material. (The treated material is not radioactive but if you have anything but the palest blue of topaz, you can assume it was so treated.)
The name topaz is ancient but may have applied to a different stone. Its name comes from either the Sanskrit tapaz, for fire, or from the ancient Greek name for an isle in the Red Sea, Topazios, now Zabargad or Zebirget. Even today, that isle is a source of the olive-green peridot and so the original topaz may have applied to that stone and thus the topaz of the bible may not be the topaz we think of.
Topaz is a hard stone (8 on the hardness scale, just below corundum, i.e., ruby and sapphire). It is naturally clear, pale blue, yellow or golden/reddish-orange. The latter color is considered the most valuable and the one for which it is still most prized. It is fairly dense at about 3.5. Coupled with its hardness, it is often found as streamworn pebbles. There’s even a ranch in Texas where you can pay to dig topaz from an old streambed. Gem quality crystals can be very large. The US Smithsonian has several crystals over 100 pounds in weight and the largest faceted gemstone in the world is a topaz, at over 36000 carats, or 16.2 pounds!
Clear topaz has been mistaken for diamond but it has a significantly lower index of refraction and about one third the dispersion (what gives diamond its fire). As with most colored gemstones, it is prized for its color and the fact that flawless crystals are fairly easy to find. With its hardness of eight, it doesn’t scratch in household dust (which aside from hair, skin and other organics typically contains sand or quartz which is hardness 7) so it makes a durable gemstone. Like diamond, it has good cleavage so care should be taken not to knock it around. It is a fairly complex mineral of aluminum, silicon, fluorine and oxygen and hydrogen.
Topaz forms late in magma crystallization, often lining cavities in rhyolites, granites and the like. From its initial source, it often weathers out into placer deposits. Russia, Brazil and Nigeria are major producers but it is found in many localities around the world and several states in addition to Texas in the US. It is sometimes confused with citrine (yellow quartz), yellow garnet or yellow sapphire.
Topaz is the birthstone for November (with Tiger’s eye) and has a number of reputed healing properties in the crystal healing community.
Treated topaz is very cheap, about a dollar a carat. For those interested in a sample, it is simple enough to acquire a few specimens of the less prized colors. If you ever go to a gem and mineral show, you may see large bins of blue topaz containing tens of thousands of carats.
For the fantasy writer, we’re always free to do whatever we want when it comes to large gemstones but for those who want to stay close to Earth, topaz makes a great candidate for the sort of fabulous gemstones that might serve as the eye of huge idol or other large scale site.