get them check out firstly..and fresh lemon juice might help...
It may be a fungal infection, get that looked at gray is not a normal, healthy toenail color
is it infection or discoloration from dark nail polish?
Lead poisoning can cause gray stridation on nails. A lot of things can cause lead poisoning, including drinking a lot of water straight from the tap. Try filling water pitchers, letting the water sit for a while, and drinking out of the pitcher.
Eww! I agree with r8r4
Dont you ever cut your toenails? Damn sheet cutters....