Huh... I actually didn't realize there was a correlation between nail strength and testosterone before reading this thread.
I will say that (from the other side of things), pre-T, my nails were weak as hell. They would bend (no splitting) and any flick to them hurt pretty bad. My fiancée's nails are harder than diamonds, though. She takes a multivitamin and a relatively high amount of vitamin D3 daily.
I, too, started taking D3 (3k mg's) and just before starting HRT my nails reached the strength of hers. A lot of people have a deficiency in that particular vitamin (especially in the winter months or the indoor types) so it could possibly give you some benefit.