User blog comment:UnarmedGamerB1skuit/how rare is diamond in terraria/@comment-4674838-20130106202648

In my experience, I have found more diamonds in or near the underground jungle than anywhere else. They are very, very rare, so I wouldn't really bother searching for them. You should just focus on other things and as you adventure you may find a few on accident. You don't really need diamonds for anything anyways.