No, I disagree, hydrothermal is most definitely a class of metamorphic, even if the water is from magmatic sources. In that case, it’s a special kind of contact metamorphism, in other cases, a special kind of regional metamorphism. But you’re right that most of the really nifty “Rock Crystals” one finds at rockhound tables are either from hydrothermal veins or pegmatites. A lot of Amethyst (Purple Quartz) comes from geodes, though, either in volcanic rocks or agate nodules.
Jinx, an oreis any rock that is mined economically. It may be a single mineral rock or a multi-mineral rock. Ores of all 3 rock types occur.
I believe some of the confusion might be because some rocks and small stones are almost all pure quartz. So a lot of people refer to those kinds of rocks as quartz, and then people think that quartz is a type of rock. Quartz isn’t a type of rock, but it is a very common constituent of rocks.
Is there a way to tell if an object is pure quartz crystal, as opposed to having lead or glass in it? For example, if you were looking at a supposedly crystal something at a yard sale, is there anything about the look of it or how it refracts light or anything you could do to authenticate it, short of taking it to a jeweler? For that matter, how does a jeweler distinguish between quartz, cubic zirconium, diamond, and other clear materials?
Clear materials in general can be distinguished both by their index of refraction and by their dispersion properties (how much the index of refraction varies for different colors). Both of these are extraordinarily high for diamond, so it’s relatively easy to pick out (for a trained jeweler, at least), but the same principles apply for any of them.
Note that quartz dissolves in hot water. It grows just like rock candy or alum crystals. Trouble is, the water has to be around 400C degrees before quartz dissolves. To keep it from boiling, you need a pressure tank at 25,000 pounds/sqin pressure.
Heh. You can heat quartz up to white hot without melting. Glass melts at orange or yellow heat.
Quartz should also scratch glass. But I’m not sure why someone would be making counterfeit quartz crystals, since they’re very common and really only valuable because they’re cool looking.
I was thinking a small quartz crystal ball would make an interesting decoration and conversation piece but I’d want to know it’s the real thing and not just glass. It would be easy for a seller to claim it is when it really isn’t (and they probably wouldn’t allow me to do any scratching or melting tests :dubious:). I guess the only answer is to try to determine the reputation of the seller.
I was actually going to suggest doing the thing where you put the ball between two polarizers at right angles, but it can be tricky to actually produce an interference figure. It’s a neat party trick, but probably not a reliable identification technique.
