What Causes A Large Black Scab On The Tip Of Toe?

I have a large black scab on the tip of my left toe. It started out as a small dot and grew from there. I was on anti bionics through a PIC line and it seemed to arrest it but, It's been nearly four months and shows very little progress. It is also started to grow on the tip of my right toe.
Yours Respectfully
Bob