# Classes of Rock: What is Quartz? **URL:** \ **Category:** Factual Questions\ **Created:** [February 23, 2010, 2:11am UTC](https://boards.straightdope.com/t/classes-of-rock-what-is-quartz/530066 "2010-02-23T02:11:45Z")\ **Posts on this page:** 10\ **Page:** 2
**Author:** ![MrDibble](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mrdibble/32/114_2.png) [@MrDibble](https://boards.straightdope.com/u/MrDibble)\ **Post date:** [February 26, 2010, 1:48pm UTC](https://boards.straightdope.com/t/classes-of-rock-what-is-quartz/530066/21 "2010-02-26T13:48:35Z")
> [@GreasyJack](#): > > (hydrothermal rocks don’t really fit into the igneous, sedimentary, metamorphic categories-- they’re rocks that precipitate out of water that is derived from and/or altered by magma bodies) 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 [ore](http://en.wikipedia.org/wiki/Ore)is 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. ---
**Author:** ![Polycarp](https://avatars.discourse-cdn.com/v4/letter/p/82dd89/32.png) [@Polycarp](https://boards.straightdope.com/u/Polycarp)\ **Post date:** [February 26, 2010, 10:31pm UTC](https://boards.straightdope.com/t/classes-of-rock-what-is-quartz/530066/22 "2010-02-26T22:31:53Z")
> [@Jinx](#): > > These jokes are very shale-low! That isn’t gneiss! 😛 ---
**Author:** ![Isamu](https://avatars.discourse-cdn.com/v4/letter/i/7c8e57/32.png) [@Isamu](https://boards.straightdope.com/u/Isamu)\ **Post date:** [March 1, 2010, 3:54am UTC](https://boards.straightdope.com/t/classes-of-rock-what-is-quartz/530066/23 "2010-03-01T03:54:46Z")
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. ---
**Author:** ![NetTrekker](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/nettrekker/32/2909_2.png) [@NetTrekker](https://boards.straightdope.com/u/NetTrekker)\ **Post date:** [January 20, 2011, 2:05am UTC](https://boards.straightdope.com/t/classes-of-rock-what-is-quartz/530066/24 "2011-01-20T02:05:46Z")
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? ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [January 20, 2011, 3:43am UTC](https://boards.straightdope.com/t/classes-of-rock-what-is-quartz/530066/25 "2011-01-20T03:43:08Z")
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. ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [January 20, 2011, 4:46am UTC](https://boards.straightdope.com/t/classes-of-rock-what-is-quartz/530066/26 "2011-01-20T04:46:52Z")
> [@LouisB](#): > > Quartz is SiO2 (Silicon Dioxide) and is one of the most common minerals on Earth. It is impervious to all acids except sulfuric, IIRC, and is found in both right and left handed formation. It exists in two forms, alpha and beta quartz; alpha quartz is piezoelectric while beta quartz is not. If alpha quartz is heated to 573 degrees C, it transforms into beta quartz. It melts at ~ 1700 degrees C. Quartz is reasonably impervious to Sulphuric acid - it’s Hydrofluoric acid that will dissolve it. ---
**Author:** ![wbeaty](https://avatars.discourse-cdn.com/v4/letter/w/439d5e/32.png) [@wbeaty](https://boards.straightdope.com/u/wbeaty)\ **Post date:** [January 20, 2011, 5:00am UTC](https://boards.straightdope.com/t/classes-of-rock-what-is-quartz/530066/27 "2011-01-20T05:00:18Z")
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. > [@NetTrekker](#): > > Is there a way to tell if an object is pure quartz crystal, as opposed to having lead or glass in it? Heh. You can heat quartz up to white hot without melting. Glass melts at orange or yellow heat. ---
**Author:** ![GreasyJack](https://avatars.discourse-cdn.com/v4/letter/g/df705f/32.png) [@GreasyJack](https://boards.straightdope.com/u/GreasyJack)\ **Post date:** [January 20, 2011, 5:11am UTC](https://boards.straightdope.com/t/classes-of-rock-what-is-quartz/530066/28 "2011-01-20T05:11:41Z")
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. ---
**Author:** ![NetTrekker](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/nettrekker/32/2909_2.png) [@NetTrekker](https://boards.straightdope.com/u/NetTrekker)\ **Post date:** [January 21, 2011, 3:05am UTC](https://boards.straightdope.com/t/classes-of-rock-what-is-quartz/530066/29 "2011-01-21T03:05:25Z")
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. ---
**Author:** ![GreasyJack](https://avatars.discourse-cdn.com/v4/letter/g/df705f/32.png) [@GreasyJack](https://boards.straightdope.com/u/GreasyJack)\ **Post date:** [January 21, 2011, 3:22am UTC](https://boards.straightdope.com/t/classes-of-rock-what-is-quartz/530066/30 "2011-01-21T03:22:27Z")
Here’s a page that has a description of ways to tell if you’re dealing with a real quartz crystal ball, of all things: [Rockhounding Arkansas, The Page you are looking for has moved](http://www.rockhoundingar.com/askmikey/askMqtz03.html) 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. [Previous page](https://boards.straightdope.com/t/classes-of-rock-what-is-quartz/530066.md?page=1)