1. You should add code to keep diamonds from spawning in rocks. It'd be simple to implement: Check if the diamond spawn location is rock, and if it is, don't spawn it.
2. Don't use a dialog for user name entry; use a form or something instead. In my browser (Chromium on Linux Mint), the dialog locks the entire browser window until I type in a name, even if I don't want to play -- maybe I just want to click on About to see more about you.
3. Is this open-source? It's probably way too simple to be commercially viable, but it'd be instructive for learning socket.io to look at the source; you should put it on Github or something.
4. Just checked the site again; it's down. The front page of HN will do that. Maybe you could throw some screenshots up on a static http server or Amazon S3 if you the ingredients handy, so people have something to look at.