Ebony would be quite easy to mod into the game. Copy the code from a tropical tree, raise the density to over 1000, change the prefstring, add to the raws, regen a world, then embark. Or you can simply use the two DF equivalent species (blood thorn and glumprong).
Edit: In fact, here's one on the house for you (spoiler for space saving):
[PLANT:EBONY]
[NAME:ebony][NAME_PLURAL:ebonies][ADJ:ebony]
[USE_MATERIAL_TEMPLATE:STRUCTURAL:STRUCTURAL_PLANT_TEMPLATE]
[BASIC_MAT:LOCAL_PLANT_MAT:STRUCTURAL]
[USE_MATERIAL_TEMPLATE:WOOD:WOOD_TEMPLATE]
[STATE_NAME:ALL_SOLID:ebony]
[STATE_ADJ:ALL_SOLID:ebony]
[PREFIX:NONE]
[STATE_COLOR:ALL_SOLID:BLACK]
[DISPLAY_COLOR:0:0:1]
[TREE:LOCAL_PLANT_MAT:WOOD][TREE_TILE:6]
[PREFSTRING:fine texture]
[PREFSTRING:smooth finish]
[DRY]
[BIOME:ANY_TROPICAL_FOREST]
[SAPLING]
[SOLID_DENSITY:1200]
[TREE_COLOR:0:0:1]
[DEAD_TREE_COLOR:0:0:1]
[SAPLING_COLOR:0:0:1]
[DEAD_SAPLING_COLOR:0:0:1]
Mithril is not in the game standard, but spoilerite contains many of the same physical properties. If you want something different, you can add the tags [ITEMS_WEAPON], [ITEMS_WEAPON_RANGED], [ITEMS_AMMO], [ITEMS_DIGGER], and [ITEMS_ARMOR] to platinum. You won't even need to regen the world you're using to make use of these new tags.