Can diamonds really cut glass?

Yes, diamond can cut glass. As a matter of fact, diamond are over 160 time harder than glass. (As the hardest substance known, diamonds can cut anything.) However, if you are thinking of using this as a test to tell if a stone is really a diamond, then I would not recommend that you try it. There are a couple of reasons why this is a bad test. First of all, as Jennifer R wrote, there are many other things that many other things that are harder than glass. The other reason why this is not a good test for a diamond is that diamonds have cleavage (inherent planes of weakness within a mineral's structure). Therefore, if you try to use a gem diamond to cut glass, you could chip the stone along a cleavage plane. So, not only would this not prove that you had a diamond, but you could damage the stone in the process.

*********
"Diamond is 10 out of a possible 10 on the Mohs hardness scale."

Yes, it is. However, Mohs hardness scale is not a linear scale. When making the scale, Mohs basically picked 10 minerals and defined them as specific values ranging from 1 to 10. They were not ranked on their absolute hardness (their actual hardness). These were just ranked on there relative hardnesses [gypsum (2) is harder than talc (1), but softer than calcite (3)]. To find a graph showing the absolute hardness of the various index minerals go to http://csmres.jmu.edu/geollab/fichter/Minerals/mohs.PDF and http://www.galleries.com/minerals/hardness.htm to get the actual numerical values of their absolute hardness.

I think what you're wanting to know is if you could actually use a diamond like they show in cartoons to cut a piece of glass. No, you can't. You can, however, use a diamond to scratch the surface of a piece of glass where you want it to have a clean break. The scratching will weaken the surface of the glass just enough that it will break clean along the line you scratch. You have to actually break the glass by supporting it near the scratch along one side, pick up the sheet of glass, then drop it to make it shear along the weak (scratched) point. I've done this many times using a carbide-tipped glass cutting tool. There are, however, diamond sawblades that are metal impregnated with industrial diamond dust around their permimeter that you can put in a saw that will actually cut glass. Look for them at your local Lowe's or Home Depot.The only way to cut glass with a fancy edge is to use a power tool designed for that purpose. Any commercial glass company can show you one.

Yes they can, but contrary to popular belief, they are not the only things that can cut glass.

www.24carat.co.uk/diamondcancutglass.html

Yes

I've see it be done.