A giant diamond would evaporate in seconds.Depends on what it's made from. Could be a solid chunk of super-high density carbon out there. Basically a giant uncut diamond that wouldn't burn up very easily. Who the hell knows. There's an entire belt out there of stuff, not counting whatever bumps into something in the Oort Cloud and heads our way.
But yeah, mostly the smaller ones aren't what we have to worry about. And fortunately a lot of the bigger ones get sucked in by Saturn and Jupiter's gravity wells. We're surprisingly well protected, all things considered.
Now if only we'd stop fighting and killing each other constantly, we could get off this planet and drop the chance of total species wipe to almost 0....that'd be great.