Your Tiffany style lamp is made by taking brightly colored pieces of stained glass and joining them together with copper solder joints.
The pieces of Tiffany art glass are tightly held in place by the soldered joints. So tightly, in fact, that they don’t vibrate and resonate sound when tapped like a normal window pane or drinking glass would.
Because of this, they create a dull, muffled sound when struck, a sound interpreted as many as being plastic.
You can tell you have a real Tiffany style glass shade by the brilliance of the colors. Some glass will have texture to it, and you may be able to see slight imperfections and irregularities in the glass. Finally, the art glass shade will be much heavier than a plastic one.
The copper soldering technique is the same time-tested, hand-made process developed by American artisan glassworkers over 100 years ago. So remember, these designs are made out of glass, so please handle with care!