You can tell them apart, with practice and light. This is not just for jewelry. It applies to bowls, goblets, flutes, anything crafted of glass and marketed as crystal.
True crystal has a rainbow of color caused by small amounts of lead being mixed with the glass while it is being made. It is not a coating on the outside. It does not scratch off. This makes crystal naturally heavier then true glass. It may just be a slight difference, but it will be heavier if it is crystal.
The color play is easy to see if you have natural light. Take this Swarovski bicone crystal bracelet I made:
You can see the colors, if you move it around, the colors shift too. Compared to these vintage clip on earrings which are cut glass, not crystal:
It reflects the blue under it well but does not have the color shifting property of the crystal shown above. It is also easier to see through for the same reason.
I hope this helps you identify crystal from glass. I would be interested in hearing your experience with misidentified crystal/glass. Feel free to comment below.
