Various solid substances that glitter because their surfaces are made up of planes that catch the light are called crystals. If you leave your ice cream in the freezer too long, you may find a coating of ice crystals on top.
Some people think that certain natural crystals — quartz, for instance — have mystical properties and if you wear them or hold them, they cure whatever’s wrong with you. Crystal is also the word for transparent glass made of silica, and it's also a name for fine glassware — the kind that reverberates with a nice ping when you flick your fingernail against it. Another kind of crystal is the glass cover that protects the face of your watch.