They do not respawn, however, glowing mushrooms respawn!

The ones you loot in crypts do not. Surtlings themselves will respawn, and can be farmed for cores.

they are infinite essentially. dont worry about useing them up.

You will eventually have so many cores you just wont know what to do with them. Pretty much everything becomes bountiful after awhile. I really wish they would limit the drops to help with inventory. Yes I know you can shut off auto pickup but its not the same. Knowing that killing something is always going to yield the same amount every time gets rather boring. Probably the best thing about mining is that you never know how many you will get if any at all.