David is correct although naturally some just aren't as dark as others or won't get as dark, and it can take a some time.
There are ways to "ebonize" woods, that are quite different than staining, more like a patina. A simple solution of steel wool dissolved in vinegar works well on many woods, but to varying degrees. Be sure to wash the steel wool with soap and water and ring it out a few times to get rid of any oil. It usually takes about 5-7 days for the solution to reach full strength. Also, if it's in a jar with a lid, put a tiny hole in the lid so it can gas out. I just tried some on a small sample of ironwood and it worked quite well and very fast. I'm not a chemist, so don't quote me on this, but I think remember reading that the iron in the acidic solution reacts with the tannins in the wood,… which is why a diluted solution of ferric chloride also works. However, if you use ferric chloride, it would probably be a good idea to neutralize it afterwards. The traditional solution of vinegar and steel wool does seem to work quite well on desert ironwood.
Here’s a shot of the sample from a few minutes ago:
Of course, the surface of the wood needs to be raw, no oil, wax or stain. It’s also a good idea to raise the grain a couple times first for a nice smooth finish, because the solution will raise the grain if you don’t.
For more information, just Google “ebonizing wood”.