A rhinestone is a cheap, sparkly stone made to look like a diamond. Now it is usually glass, crystal glass, or plastic with a shiny back, sewn or stuck onto clothes, shoes, phone cases, and stage jackets. Though actually, the first ones were not plastic at all: they were little bits of rock crystal linked with the Rhine, and French called them “Rhine pebbles.” English simply copied that idea as Rhine + stone. So a rhinestone is not a diamond, not a precious stone, and not from an animal. It is honest fake sparkle — used because it catches the light and does not cost much. On a concert costume that loud shine is the point; in a jewellery shop it can sound like a polite way of saying “not real.”