a-cool-rock

Suppose we were walking outside, and you bent over and picked up what you would describe as "a cool rock" before showing it to me. What kind of rock are you likely to have picked up?

• A particularly round one~
• A remarkably smooth one~
• A crystal-structure type rock, probably a mineral~
• An especially flat one~
• A monochrome one, perhaps jet-black or pure-white~
• There's a specific kind of cool rock I pick up, and you didn't list it here~