Diamond Pick

Listen up, buttercup. This "joke" is trying to be smooth. The speaker is comparing a girl to a diamond pickaxe from Minecraft. Obsidian is a dark glass-like volcanic rock that is known to be hard, and the speaker is saying they are as hard as it. You probably have never left your mom's basement, so I am sure you don't know anything about the outside world. So basically he is saying he has a boner.