The song Diggy Diggy Hole makes some questionable assertions about dwarven anatomy. Specifically:
Suckled from a teat of stone
Raised in the dark
The safety of our mountain home
Skin made of iron
Steel in our bones
To dig and dig makes us free
Come on, brothers, sing with me!
Now we could interpret that all metaphorically... but where's the fun in that?
So, how do we design a dwarf that actually meets those criteria?
"Skin made of iron" might have something to do with iron sulfide scales or similar. And I am willing to posit that the "teat of stone" may be merely superficially similar to stone, rather than literally made of rock. But the closer an answer gets to a straightforward literal interpretation of the song, the better.