Gems and Stones A Quartz by Any Other Name….
You know all about quartz – it’s a common mineral species with a chemical composition of SiO2. But is there more? Quartz gems can be fully crystalline or crypto-crystalline in structure.
The fully fully crystalline varieties include the well known rock crystals; citrine; amethyst; smoky quartz or cairngorm (light to dark brown) to morion (nearly opaque black) in color; rose quartz; aventurine quartz; iris or rainbow quartz; rutilated or tourmalinated (sagenitic) quartz; dumortierite quartz; milky quartz; gold quartz; star or asteriated quartz; green quartz, praseolite or greened amethyst; and quartzite. Quartz may exhibit chatoyancy or cat’s eye effect. Tiger’s eye is actually a quartz pseudomorph after crocidolite, where the original material of a blue asbestiform variety of riebeckite has been altered by decomposition into silica.
Misnomers for rock crystal include: Herkimer diamond, Arizona diamond, Alaska diamond and Cornish diamond. Smoky quartz has also been called smoky topaz, burnt topaz or Scotch topaz. Dumortierite quartz has been called California lapis. Semi-transparent milky quartz has been called moonstone and green aventurine quartz has been called Indian jade.
Then we get into the chalcedony group (that’s pronounced “kal-SED-uh-nee".) which is quartz with a micro-crystalline or crypto-crystalline structure.
The variety names in this group include milky chalcedony which is semi-transparent (ST) to translucent (TL), nearly colorless or white; chrysoprase, a ST to TL light to medium yellowish green; prase, a TL to semi-TL, generally darker and less saturated green than chrysoprase; bloodstone (also known as heliotrope) which is dark green with red to brownish red spots.What’s the difference between carnelian and sard? Carnelian is a ST to TL yellow-orange to orangy red, brownish red or brownish orange, while sard is a ST to TL dark brownish red to brown to dark orange. Sard is darker and the color is less saturated than carnelian. Oh, but wait, we have onyx which is composed of straight, parallel layers of different colors; sardonyx – sard colors alternating with bands of either white or black; and carnelian onyx – onyx showing bands of carnelian colors alternating with bands of either white or black.
Confused yet?
We all know agates – whether it’s the general banded agate, landscape agate, picture agate, fortification agate and agates of umpteen various localities. And there is moss agate (also known as mocha stone) – a chalcedony with green, black or brown inclusions that look like moss. Iris agate is ST to TL with iridescent colors. Fire agate is STL to opaque (O) with iridescent colors against a brown body color. Dendritic agate is a chalcedony with dark inclusions resembling tree branches.
There is also the beautiful chrysocolla chalcedony or chrysocolla quartz – TL to STL, intense light blue or blue green, colored by the copper mineral chrysocolla.
Have you heard of “plasma”? This one is STL to opaque dark green microgranular or microfibrous chalcedony with white or yellowish spots. What about amethystine chalcedony – also marketed under the name damsonite?
Chalcedony is also a fossilizing agent. Wood that has been entirely replaced with chalcedony can be called petrified wood, agatized wood or silicified wood. The petrified wood often resembles a patterned jasper.
So what is jasper? Jasper is a general variety term applied to STL to O chalcedony of any color or combination of colors unless it has a specific variety name. Jasper, unlike agate and chalcedonies, can occur as an extensive bed of sedimentary or metamorphic origin. Jaspilite is a metamorphic rock containing alternating layers of jasper with black or reddish hematite. Jasponyx (band jasper or ribbon jasper) is a thin-banded jasper with alternating dark or light bands. Lydian stone or touchstone (basanite) is a black, fine-grained jasper that is used for testing the streak of gold alloys.
Misnomers of chalcedony include Swiss lapis or German lapis (dyed blue Jasper), moonstone (milky chalcedony), black onyx (almost always dyed) and green onyx (dyed).
Now that my spell checker is confused, my head is spinning and you have all these new terms in your vocabulary, it’s time to stop.
Janice
Did you know? That Tanzanite has been named as the third birthstone for the month of December along with turquoise and zircon? Key industry organizations including the American Gem Trade Association (AGTA), International Colored Stone Association (ICA), and the Jewelers of America (JA) signed on to the proposal. Tanzanite is the first gem to be added to the birthstone list since 1912. Tanzanite is said to be the second most popular colored gemstone in the U.S., behind sapphire.