Kimberlite
Y'know, the stuff that holds diamonds. They would appear as a much darker form of plain stone. They would be mined with an iron pick. Smelting them would have a 1 in something chance of yielding diamond. They would appear as small pipes leading deep underground. Be careful, because magma sometimes hides in and under the pipe.
Anvil
Adds durability to worn down iron tools. To make it, place 2x iron in a T formation on the 3x3 crafting bench. The anvil "interface" would just be one square for iron tools. The process is twice as long as smelting. Once it's complete, the tool would have most of it's durability back, saving materials for other iron tools. Once the tool had it's durability added, it can't be added again.