Fire agate

[terriejay]

I bought some of this at the last trunk show.  

I've seen fire agate before, but this is very different since it's milky and has no "fire" but does have very interesting patterns.  I've been trying to learn more about it but there doesn't seem to be any information out there.

What can you tell me about it?  Where does it come from?  Is it natural or enhanced?  What causes the snakeskin-like patterns?

[amyonlyone]

This is what I've been able to find out about Fire Agate.

 

Chemistry   SiO2
CRYSTALLOGRAPHY Hexagonal
REFRACTIVE INDEX 1.544 - 1.553
HARDNESS 7
SPECIFIC GRAVITY 2.651
CLEAVAGE None
HEAT SENSITIVE No
WEARABILITY Excellent

Agate is probably the most common gemstones. This variety of silica is generaly formed by volcanic activity. Fire Agate is thought to be formed when hot water saturated with colloidal silica and iron oxide. Invades cavities in regular rock below the earth's surface.

When it begins to cool. Chalcedony with iron oxide begins to grow on any available surface. The solution grows layers of silica and iron oxide. Many shapes and colors are formed by the mineral impurities in the silica.

The alternating silica and iron oxide layers are called Schiller. The Schiller layers are small enough that light passing through them. Forming an interference of colors known as Fire.
These impurities give fire agate the distinction of "Gem", rather than just agate.

As the iron oxide in the solution runs out, colorless chalcedony continues to grow. Producing the typical brown and white rough fire agate. The iron oxide is what gives this gem it's basic brown color.

Some of the most brilliant fire agates are from Arizona. The micro-thin layers in this material, diffract light, back to the eye in rainbow patterns. Creating brilliant bubbles and sheets of red, green, yellow, and the elusive blue. From  mines such as, Deer Creek, Slaughter Mountain and others.
Hope this helps.

[Russ Nobbs]

I just noticed this post from last month.
"crab fire agate" is another one of those descriptive names that are used in the bead industry.  This stone is really a kind of carnelian that may have an enhanced surface pattern. (Jury is still out on that part I think.)

See: http://www.mindat.org/mesg-55-39224.html
and the thread at Bead and button Forum at http://www.beadandbutton.com/bnb....D=25193
The fire agate topic is down on the 5th or so page.

Spiderweb carnelian might be a better name. Carnelian is, of course, a kind of agate, which is a sub group of chalcedony.

This particular stone is NOT related to the classic Mexican fire agate from Agua Caliente as the name implies. Sorry for the confusion - we use the names our vendors give us and often the Chinese descriptive name is misleading.