At first glance, quartz crystals and glass can look like identical stones. There are notable differences in their internal structural composition, but it takes lab equipment and analysis not available to the average person to differentiate the two materials on this basis. Fortunately, there are other ways to determine whether a stone is quartz crystal or just glass.
Visually inspect the suspect stone. If it is glass, perfectly round air bubbles are usually seen, with or without the aid of a jeweller's 10X loupe. 10X means the item is magnified 10 times larger than normal. To properly use a jeweller's loupe, hold the loupe snugly against your face right in front of your eye. Without squinting, move the specimen closer to the loupe until you can see it clearly. Inspect the specimen for air bubbles. If air bubbles are present, it is glass, not quartz crystal. Quartz crystal may contain imperfections, but they are not perfectly round like air bubbles.
Perform your own Mohs hardness test. Quartz crystals are harder than glass. A hardness scale invented in 1812 by German geologist Friedrich Mohs is used for these materials. Glass rates around 5.5 on the Mohs scale. Quartz crystals rate around 7 on the Mohs scale. Therefore, a piece of quartz crystal should easily scratch a piece of glass. Test the unknown stone under inspection by trying to scratch a common piece of glass such as a glass bottle with it. If the object easily scratches the glass, it probably is quartz crystal. If it takes a lot of effort to scratch the glass, it likely is another piece of glass.
If you have access to a gem tester, measure the thermal conductivity of the suspect stone by pressing the gem tester probe gently but firmly against it. Unlike natural gemstones, glass is an insulator, so it does not conduct heat well at all; consequently, if the indicator needle stops at the lowest reading on the gem tester scale, that one is labelled "Glass." If the suspect stone is Quartz crystal, however, there is some heat conductivity and the gem tester indicator needle moves to the area labelled "Quartz, Amethyst, Citrine" on the scale.
Use caution when performing a scratch test; as the specimen is damaged in the process, this clearly affects its appeal and value. Try to perform the test in an inconspicuous area and make as small of a scratch as possible.