I have a clear gemstone that I think is a diamond but fear may be cut glass. How can I tell without taking it to a jeweler?
What you have to do is sort out what properties distinguish diamond from glass, and there are quite a few you can use. You also would want to use a “non-destructive’’ test, meaning that you would like the stone to survive the test regardless of whether it turns out to be a real diamond.
One simple test is to exploit the fact that the speed of light in glass is about two-thirds that of light in air - and similar to the speed of light in clear corn syrup. That speed difference is what allows you to see glass in air, because some light reflects from the air-glass interface, where the speed changes. If you drop a glass item into clear corn syrup, you would hardly see it because the speed of light in the liquid and in glass are quite similar. A diamond, however, would be easy to see, since light travels through it much more slowly than through clear corn syrup.
Light travels 2.4 times faster through air than through a diamond, and that radical difference is what gives a well-cut diamond most of its sparkle. If your object has a flat top face and pointed bottom, put your finger on the flat side and try to see it through the pointed end. If you can see the finger, then it’s likely glass. If you can’t, there’s a good chance it’s a diamond, with light getting bent around much more inside.
Diamonds conduct heat well while glass does not. If you put your finger against the back of a diamond and push the other face against a hot object, you will quickly get burned, as if the diamond were metal. This does not happen with glass.
You can also use the scratch test. Diamond is so hard it will scratch anything that isn’t diamond - including glass. Glass, on the other hand, can’t scratch a diamond, or other glass. Drag an edge of your stone across a piece of tough glass, such as cookware, or a hard porcelain. No scratch means it’s certainly not a diamond and it’s probably just glass.
If you don’t care what happens to the stone, you can blast it with a propane torch (1,995 degrees Celsius) or an oxy-acetylene welder’s torch (up to 3,480 C.), which will cause glass to start flowing, but not a diamond. Glasses come in a wide variety, with melting points ranging from 536 to 1,140 C. Diamond’s melting point is 3,550 C.
Ask Dr. Knowledge is written by Northeastern University physicist John Swain. E-mail questions to drknowledge@globe.com or write to Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819. 
