# Any easy way of telling whether a rock is a diamond? **URL:** \ **Category:** Factual Questions\ **Created:** [August 19, 2004, 3:40am UTC](https://boards.straightdope.com/t/any-easy-way-of-telling-whether-a-rock-is-a-diamond/260505 "2004-08-19T03:40:51Z")\ **Posts on this page:** 16\ **Page:** 1
**Author:** ![Earl\_Snake-Hips\_Tucker](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/earl_snake-hips_tucker/32/3269_2.png) [@Earl\_Snake-Hips\_Tucker](https://boards.straightdope.com/u/Earl_Snake-Hips_Tucker)\ **Post date:** [August 19, 2004, 3:40am UTC](https://boards.straightdope.com/t/any-easy-way-of-telling-whether-a-rock-is-a-diamond/260505/1 "2004-08-19T03:40:51Z")
Without getting it appraised, that is. I found a ring on the beach many years ago. It had a stone on it that looked like your basic round diamond cut. However, for all I know, it’s just a piece of glass. Not that it’s anything huge (I’d guess ½ carat), but I’d still like to know. Any way without paying an expert? ---
**Author:** ![Sue\_Duhnym](https://avatars.discourse-cdn.com/v4/letter/s/91b2a8/32.png) [@Sue\_Duhnym](https://boards.straightdope.com/u/Sue_Duhnym)\ **Post date:** [August 19, 2004, 3:47am UTC](https://boards.straightdope.com/t/any-easy-way-of-telling-whether-a-rock-is-a-diamond/260505/2 "2004-08-19T03:47:39Z")
Go to a jewelry store and ask them to use their diamond tester. It takes about a second, possibly two. Any store should have them…if you accept returned merchandise, you pretty much HAVE to. ---
**Author:** ![Polycarp](https://avatars.discourse-cdn.com/v4/letter/p/82dd89/32.png) [@Polycarp](https://boards.straightdope.com/u/Polycarp)\ **Post date:** [August 19, 2004, 3:51am UTC](https://boards.straightdope.com/t/any-easy-way-of-telling-whether-a-rock-is-a-diamond/260505/3 "2004-08-19T03:51:01Z")
I presume “your basic round diamond cut” is the standard “brilliant cut” that most diamonds are faceted with? One very quick way to tell is to take a piece of corundum, and try to scratch it with it. If the corundum scratches the stone, it’s not diamond; if the diamond scrapes the corundum, it is. Nothing (at least nothing readily available) is harder than corundum but diamond, so this is a pretty foolproof test. While a sapphire or ruby is one source of corundum, corundum sandpaper is available in most hardware stores, and should do the trick. ---
**Author:** ![Sue\_Duhnym](https://avatars.discourse-cdn.com/v4/letter/s/91b2a8/32.png) [@Sue\_Duhnym](https://boards.straightdope.com/u/Sue_Duhnym)\ **Post date:** [August 19, 2004, 4:17am UTC](https://boards.straightdope.com/t/any-easy-way-of-telling-whether-a-rock-is-a-diamond/260505/4 "2004-08-19T04:17:25Z")
> [@Polycarp](#): > > One very quick way to tell is to take a piece of corundum, and try to scratch it with it. If the corundum scratches the stone, it’s not diamond; if the diamond scrapes the corundum, it is. Nothing (at least nothing readily available) is harder than corundum but diamond, so this is a pretty foolproof test. While a sapphire or ruby is one source of corundum, corundum sandpaper is available in most hardware stores, and should do the trick. Yeah, but if you do that and it IS a diamond, you’ve ruined a perfectly good piece of sandpaper. ---
**Author:** ![ltfire](https://avatars.discourse-cdn.com/v4/letter/l/7ea924/32.png) [@ltfire](https://boards.straightdope.com/u/ltfire)\ **Post date:** [August 20, 2004, 12:08am UTC](https://boards.straightdope.com/t/any-easy-way-of-telling-whether-a-rock-is-a-diamond/260505/5 "2004-08-20T00:08:52Z")
Try giving it to your girlfriend or wife. All women are experts. 😃 ---
**Author:** ![Squink](https://avatars.discourse-cdn.com/v4/letter/s/b5e925/32.png) [@Squink](https://boards.straightdope.com/u/Squink)\ **Post date:** [August 20, 2004, 12:47am UTC](https://boards.straightdope.com/t/any-easy-way-of-telling-whether-a-rock-is-a-diamond/260505/6 "2004-08-20T00:47:57Z")
All you need is a [magnifying glass, and a test for CO[sub]2[/sub]:](http://216.239.41.104/search?q=cache:CFh4wu2eaooJ:genchem.chem.wisc.edu/lectures/lecture_docs/39/unit1_6.pdf++%22burning+diamond%22&hl=en&ie=UTF-8) > [@](#): > > Around 1790, predating the periodic table by 80 years. Tenant, an English chemist, found that burning diamond and burning coal produced the same gaseous compound. Burning a diamond requires very high temperatures: these were achieved with sunlight and giant magnifying glasses. Of course, a piece of glass will just melt on you, so you might not even need to check the combustion products. ---
**Author:** ![stockton](https://avatars.discourse-cdn.com/v4/letter/s/b5a626/32.png) [@stockton](https://boards.straightdope.com/u/stockton)\ **Post date:** [August 20, 2004, 2:43am UTC](https://boards.straightdope.com/t/any-easy-way-of-telling-whether-a-rock-is-a-diamond/260505/7 "2004-08-20T02:43:59Z")
I’m sure that **samclem** will be here shortly with a definitive answer, but in the meantime: don’t try to scratch it with something. Your mall jeweler might not even understand your question. Your local independent jeweler will probably be happy to take a look at it and give an opinion, but not an appraisal certificate. If you explain it just like you did here, there’s no problem with checking and confirming your hopes or bursting your bubble. My oldest pal is a goldcrafter in NC. He would give you the straight dope without worrying about a few bucks. After all, you’re a potential customer! ---
**Author:** ![samclem](https://avatars.discourse-cdn.com/v4/letter/s/a9a28c/32.png) [@samclem](https://boards.straightdope.com/u/samclem)\ **Post date:** [August 20, 2004, 2:54am UTC](https://boards.straightdope.com/t/any-easy-way-of-telling-whether-a-rock-is-a-diamond/260505/8 "2004-08-20T02:54:23Z")
Heck! Pretty good answers already. My only question would be, how large was the magnifying glass used by Tenant? ---
**Author:** ![Squink](https://avatars.discourse-cdn.com/v4/letter/s/b5e925/32.png) [@Squink](https://boards.straightdope.com/u/Squink)\ **Post date:** [August 20, 2004, 4:11am UTC](https://boards.straightdope.com/t/any-easy-way-of-telling-whether-a-rock-is-a-diamond/260505/9 "2004-08-20T04:11:28Z")
> [@samclem](#): > > My only question would be, how large was the magnifying glass used by Tenant? A bit more digging has convinced me that the story in my link is not correct. In 1796, [Smithson Tennant](http://homepage.mac.com/dtrapp/Elements/properties.html) found a way to prove that diamonds were composed solely of carbon by burning them with potassium nitrate. It was [Lavoisier](http://216.239.39.104/search?q=cache:-uCemCZxgHUJ:anatomy.med.unsw.edu.au/cbl/library/williams_h/science_history/4hsci.pdf+Tennant+diamond+%22magnifying+glass%22+1790&hl=en&ie=UTF-8) who, in the 1770’s, used the “great lens of Tschirnhausen” to burn diamonds, and show that they contained at least _some_ carbon. [Tschirnhausen](http://archimedes.mpiwg-berlin.mpg.de/cgi-bin/toc/toc.cgi?dir=hutto_dicti_078_en_1795;step=textonly;corpus=&page=248&n=248) made some pretty big lenses: > [@](#): > > The most considerable of this kind is that made by Tschirnhausen, and described in the same Acta Erudit. The diameters of his lenses are from 3 to 4 feet, having the focus at the distance of 12 feet, and its diameter an inch and a half. To be called great, the lens Lavoisier used would have had to be at the upper end of that size range. Not exactly a common household item, but if you happen to have one, you’re all set! 😉 ---
**Author:** ![Catalyst](https://avatars.discourse-cdn.com/v4/letter/c/e47774/32.png) [@Catalyst](https://boards.straightdope.com/u/Catalyst)\ **Post date:** [August 20, 2004, 7:14am UTC](https://boards.straightdope.com/t/any-easy-way-of-telling-whether-a-rock-is-a-diamond/260505/10 "2004-08-20T07:14:54Z")
My high school physics teacher claims that because of the particular indices of refraction of cubic zirconia and water, one can spot a fake diamond by dropping it in a glass of water; if you have trouble seeing it, it’s not a diamond. Looking at the indices of refraction for water, diamond, and cubic zirconia (1.333, 2.417, and 2.150, respectively), however, I can’t see this trick working. Can anyone confirm whether this method can actually be used to tell a real diamond from a fake? ---
**Author:** ![Colophon](https://avatars.discourse-cdn.com/v4/letter/c/f05b48/32.png) [@Colophon](https://boards.straightdope.com/u/Colophon)\ **Post date:** [August 20, 2004, 10:14am UTC](https://boards.straightdope.com/t/any-easy-way-of-telling-whether-a-rock-is-a-diamond/260505/11 "2004-08-20T10:14:07Z")
> [@Catalyst](#): > > My high school physics teacher claims that because of the particular indices of refraction of cubic zirconia and water, one can spot a fake diamond by dropping it in a glass of water; if you have trouble seeing it, it’s not a diamond. Looking at the indices of refraction for water, diamond, and cubic zirconia (1.333, 2.417, and 2.150, respectively), however, I can’t see this trick working. Can anyone confirm whether this method can actually be used to tell a real diamond from a fake? He may have been thinking of identifying glass fakes - small pieces of glass can be nearly invisible in water, with an index of refraction of around 1.5. ---
**Author:** ![duffer](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/duffer/32/1026_2.png) [@duffer](https://boards.straightdope.com/u/duffer)\ **Post date:** [August 20, 2004, 10:53am UTC](https://boards.straightdope.com/t/any-easy-way-of-telling-whether-a-rock-is-a-diamond/260505/12 "2004-08-20T10:53:23Z")
> [@ltfire](#): > > Try giving it to your girlfriend or wife. All women are experts. 😃 Wrong. She’ll be so psyched by getting the ring, no logic will apply. It’s her evil friends that will spot the fake. Giving them more ammunition to slam you. **ltfire** , you have much to learn. 😉 ---
**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:** [August 20, 2004, 5:49pm UTC](https://boards.straightdope.com/t/any-easy-way-of-telling-whether-a-rock-is-a-diamond/260505/13 "2004-08-20T17:49:54Z")
> [@](#): > > Nothing (at least nothing readily available) is harder than corundum but diamond, so this is a pretty foolproof test. What about silicon carbide? Granted, I’ve never heard of gem-quality (or even transparent) carbide, so the test is still good. On the other hand, even if the rock isn’t diamond, it might still be valuable, if it’s some other precious stone, or even if it’s just a particularly pretty piece of glass. And a scratch test with something just barely softer than diamond would damage any of those. I would try to test index of refraction or density, before using a scratch test. ---
**Author:** ![CheapBastid](https://avatars.discourse-cdn.com/v4/letter/c/8baadc/32.png) [@CheapBastid](https://boards.straightdope.com/u/CheapBastid)\ **Post date:** [August 20, 2004, 8:27pm UTC](https://boards.straightdope.com/t/any-easy-way-of-telling-whether-a-rock-is-a-diamond/260505/14 "2004-08-20T20:27:24Z")
I had a very cool link that went over the many ‘diamond-esqe’ stones and how to differentiate them from a diamond, but I can’t find it. I do remember that a good low budget way to test is to exhale on the stone. Diamonds conduct heat very well, so by the time the ring moves from your mouth to your eye you should not see any fog on the stone. ---
**Author:** ![CheapBastid](https://avatars.discourse-cdn.com/v4/letter/c/8baadc/32.png) [@CheapBastid](https://boards.straightdope.com/u/CheapBastid)\ **Post date:** [August 20, 2004, 8:35pm UTC](https://boards.straightdope.com/t/any-easy-way-of-telling-whether-a-rock-is-a-diamond/260505/15 "2004-08-20T20:35:45Z")
> [@CheapBastid](#): > > I had a very cool link that went over the many ‘diamond-esqe’ stones and how to differentiate them from a diamond, but I can’t find it. [Found it](http://www.scotgem.demon.co.uk/!moisstest.htm)! It isn’t as useful in this case as I thought, but I found it fascinating. ---
**Author:** ![Marconi\_Schmeese](https://avatars.discourse-cdn.com/v4/letter/m/ea666f/32.png) [@Marconi\_Schmeese](https://boards.straightdope.com/u/Marconi_Schmeese)\ **Post date:** [August 20, 2004, 10:06pm UTC](https://boards.straightdope.com/t/any-easy-way-of-telling-whether-a-rock-is-a-diamond/260505/16 "2004-08-20T22:06:04Z")
Take it to a pawn shop. Only be concerned if they tell you it’s fake but still want to buy it from you. Behavior not limited to pawn shops, by the way.