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?
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.
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.
Yeah, but if you do that and it IS a diamond, you’ve ruined a perfectly good piece of sandpaper.
Try giving it to your girlfriend or wife. All women are experts. 
All you need is a magnifying glass, and a test for CO[sub]2[/sub]:
Of course, a piece of glass will just melt on you, so you might not even need to check the combustion products.
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!
Heck! Pretty good answers already.
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 found a way to prove that diamonds were composed solely of carbon by burning them with potassium nitrate. It was Lavoisier who, in the 1770’s, used the “great lens of Tschirnhausen” to burn diamonds, and show that they contained at least some carbon.
Tschirnhausen made some pretty big lenses:
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! 
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.
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.
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.
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.
Found it! It isn’t as useful in this case as I thought, but I found it fascinating.
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.