I think most of that perception comes from cultural depictions of jewel thieves. If you're stealing the diamonds, not paying for them, then they're a very concealable and conveniently value-dense. It doesn't matter to a jewel thief that the tiny little shard of shiny gravel was originally purchased for many thousands of dollars. What he cares about is he can hide it anywhere and get hundreds of dollars for it. Much better than stealing TVs.