What IS an anvil? Simply a block of something tough enough to withstand the heat of a red hot piece of metal and the repeated pounding of a heavy hammer.
But the shape of an anvil is important as well, so a simple flat block of stone would not work.
How about this:
Anvils can be made of stone, copper, bronze, iron, steel and adamantine.
If you use an anvil to forge an item using the same metal that the anvil is made of, or a weaker material, it will never break.
With stronger materials however the break chance increases. The damage is not catastrophic unless the difference between the materials is very big. For example trying to forge an adamantine mace on a stone anvil would break it instantly.
If you crafted steel items on an iron anvil, the anvil would last a long time before breaking. The damage builds up over time and the break chance starts at nothing and slowly creeps up.
If an anvil happens to break, it is not thrown out, it is simply "broken". It could be repaired later or melted down and recycled.
A forge that has a broken anvil simply cannot be used and all work comes to a halt. To fix this you would have a menu when you can pick a replacement anvil for that one forge, a dwarf will then haul an anvil to that site and the work begins again,.
The metalsmith that shows up to your fort brings with him an iron anvil. This should be all you need. In an emergency or if the anvil gets lost you can temporarily replace it with one of a weaker metal or even one made out of stone.
Also, if you have not hit iron yet and want to train multiple smiths at the same time, you could create other anvils of bronze or copper, because these metals will be abundant at the start.
Iron is rare and hard to get and losing your only anvil should not kill your game. So how does that sound?
[ May 14, 2007: Message edited by: Tamren ]