I think some girls like its sparkliness, its hardness and the beauty they get when worn. As for me, I don't care so much about beauty, nor do I care so much about diamonds, but some other things. Diamonds aren't exactly my BEST friend, but I still think it's pretty. I think diamonds are mostly a girly-girl's best friend, or at least someone who isn't a heavy tomboy. There are exceptions, but that is what it is the majority of the time.
Theyre only best friends of girls who don't realize what happens to people because of diamonds. You should read the book "a long way gone memoirs of a boy soldier". You might just think a bit differently about diamonds after that.