What does this diamond symbol represent?

[reed]

I've searched all over and I can't figure out what this means:

It's the orange diamond to the left of @.

[EpicMan]

It's buried treasure (i.e. money in the wall). It's plain ASCII symbol is a '*'.

You can dig it out with the 'T' command (Control-t if you're using the Roguelike (i.e. laptop) keyset); you can actually dig through any wall except permanent rock (like the walls on the edges of the levels), if you didn't know. Digging goes fastest if you wield a digging tool like a shovel or pick, but it's not too bad if you have high Strength and a reasonably heavy weapon.

You will also find money lying around or dropped by monsters you kill, so digging them out is strictly a luxury.

[Maddz]

You may want to avoid too much tunnelling while your character is low level - the longer you spend tunnelling, the more monsters respawn.

[Derakon]

Use the 'l'ook command to examine your surroundings; that tile would have a description something like "a magma vein with treasure".