Amber French or Black English?

[Mike R]

"Water of hydration?" In a nonporous stone? Help me understand how that works?

Scientific Properties:
Mohs Hardness of 7 with a trigonal crystal structure.

""Opal is a mineral species which is amorphous in structure, (without form). It has a chemical composition of SiO2.nH2O. (water content usually 3 to 14% by weight, but sometimes as high as 20%). It is transparent to opaque in appearance and is usually seen cut into cabochons, carvings, and beads and is occasionally faceted. Refractive index is around 1.450 but can go as low as 1.42 depending on the type of Opal. The specific gravity is 2.15 and the hardness is soft, being a 5. Opal is very sensitive to heat or sudden change in temperature. Never clean your Opal jewelry in an ultrasonic cleaner, it will crack the Opal.

In other words...opal is created of tiny spheres of amorphous silica gel, which gives it its "Fire" and glittery play of colors. A non-crystalline stone, it is possible for the opal to dry out and crack. It is a delicate stone, and can have as much as thirty percent of it's body weight in encapsulated water. This too explains why too much heat can cause an opal to quickly lose it's luster, and why a crack or craze to the surface may occur by moving from a hot room to the cold outdoors.""

The way that flint nodules are formed and the fact that the chemical composition is similar results in similar properties.

Besides, were we talking about oil-soaked flints?

Perhaps you were, but others were discussing the absorption of water and how nodules are often found in water pockets.  Further to answer your oil soaked flint question. flints were often stored in coal oil to prevent them from further drying out and it was then believed that oil soaked flints gave a hotter spark.  Coal oil is not kerosene.

Help me understand how that works?
I'm old and easily confused.

There are people who can help you with that.  Unfortunately, I'm not one of them.

Flint/chert is typically micro- or crypto-crystalline, whereas opal is not --it is amorphous as you state: both are silica [SiO2].  Water may be held in such materials in several ways, e.g., as water in micropores or as "bound water" at the molecular level--typically opal has more water than chert/flint. Small amounts of opal may be present in some flint--the origins of these "minerals" [strictly speaking, opal is not a mineral because minerals have crystal structure], is varied and in some dispute, but most likely is from the solidification of organic silica [as from microorganisms tests] masses originally deposited in water--many cherts/flints have fossil tests in them. Water was taken up or trapped at time of deposition.  I still maintain that the chief difference in performance is flint cut and lock geometry--not to say that certain flints, due to natural variability cannot be better than others, just that color alone does not seem significant.

[rich pierce]

Mike,
I saw a cool nodule Sunday with a horsetail fossil in it and the rock here in Missouri often has mollusk fossils as well.  Since it formed here as inclusions in the limestone I guess that's bound to happen.  The quality of local rock varies widely even within a "seam".  I believe "chalk flint" from deep in the ground is the best for ease of knapping.  But it's fine grain means that it must be sharp to make sparks.  I think it's possible the grain of rougher chert can grab the steel of the frizzen even when a little dull.  It is tough stuff to knap though.  Sunday I found a tough piece that yielded flints I bet would be good for 200 shots in a good lock.

[Daryl]

Send 'em to me, Rich!!!! HA!

[Mike R]

Mike,
I saw a cool nodule Sunday with a horsetail fossil in it and the rock here in Missouri often has mollusk fossils as well.  Since it formed here as inclusions in the limestone I guess that's bound to happen.  The quality of local rock varies widely even within a "seam".  I believe "chalk flint" from deep in the ground is the best for ease of knapping.  But it's fine grain means that it must be sharp to make sparks.  I think it's possible the grain of rougher chert can grab the steel of the frizzen even when a little dull.  It is tough stuff to knap though.  Sunday I found a tough piece that yielded flints I bet would be good for 200 shots in a good lock.

Cool.  Yes, larger fossils are not rare in bedded cherts--and often well-preserved in detail in the silica.  Bedded cherts differ in some ways from the nice English nodules in the chalks there--and are more variable in texture.  Arkansas has enormous amounts of bedded cherts called novaculite that was used by natives for points and tools and still used for whetstones. Even in this "one formation" significant variation occurs in physical properties of the chert.  But English nodules have less variation and only the 'best' [most usable] are knapped.  Even s0 we have all seen variation in black flints--some are vitreous and black, others grainier and grey, etc.,, Some have nice tables, some have central peaks--that is they are "knapped" differently.  The MO cherts I have from Rich are dense and nearly white.  Novaculite can be white, black , red, gray, etc.  The whetstone classes, "Hard", "Soft" and "Washita" reflect not hardness but porosity--which appears to the naked eye and "feel" as graininess. All are the same hardness [7].  Opal by the way is not as hard as chert--being noncrystaline.