How about this:
Adamantine is present in small pockets very deep underground. Every now and then a large bubble of adamantine can be found. Most are empty, but some contain hidden treasure, and most often, demons. If you delve even deeper you may find a gigantic pocket containing a greater demon.
In game terms, feep past the magma you will start to find scattered pockets of adamantine, most of these pockets are the size of gem clusters. Every now and then you will find a larger deposit. Invariably each of these has a demon pit inside (with a finite number).
If you want to avoid the demons you can carefully skim tiles off the sides of the deposite and leave the pit encased. Or if you have enough manpower, you can break open the bubble and kill the demons. Since Z levels are in, we could possibly dig out the demon pits and see what they keep in there, probably treasure. This goes on for many layers. However once you get VERY deep you will find larger, thicker bubbles with lone but very powerful demons.
Eventually you hit this solid wall of adamantine. This can be mined to your hearts content. HOWEVER! The wall has cracks in it and is not of uniform thickness. If you happen to bust through the wall, then all hell breaks loose. Or rather, the uberdemon. If you somehow defeat him then you can go ahead and mine the adamantine to your hearts content, but good luck with that!
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Now this way we can gather adamantine without making the game die. There are considerable risks involved which balances out the rarity. For one you have to dig DEEP, you may uncover caves full of beasts or lava pockets.