Sometimes a cubic zerconuim or cut glass can be sold to a customer as a diamond. Here is how you can tell the difference between a real diamond and a cubic zeroconium or cut glass.
The main differences between glass and diamonds or cubic zirconia is that glass is much softer then either of these materials. If the stone easily gouges glass, it is either a diamond or a cubic zirconia. If it doesn't at all, it is glass.
Cubic Zirconia is softer then diamond. Cubic Zirconia also has a higher specific gravity (5.95) then a diamond (3.52). But this can only be easily measured if you have the right equipment.
A simple way to distinguish glass from cubic zirconia from diamond is to use a black light. In a black light diamonds will usually glow blue, a cubic zirconia will glow mustard yellow, and glass will not glow at all.
