Take gold, for instance. Sure, it's kinda sorta pretty. But it's heavy and it's not very useful except as an electrical conductor. And I know from experience it gets worn down pretty easily. I have a gold
Then diamonds. I can't tell the difference between a diamond and a piece of cut quartz. They look exactly alike. Why are diamonds prized? They're not especially pretty, even. The only thing they're good for is drill bits and other cutting things, being as they're the hardest known natural substance. But yeah, they're not pretty. I prefer colored gems. Emeralds I can understand.
Those two things are all I can think of right now. But I'm sure there are other things.
* Duh, WTF? COIN?