Hmmm....
I would wonder why it isnt happening to any other finger. Do you have excema or dermatitis?? Have you tried applying a hydrocortizone product?
I use hot stones every day as well and have a problem occationally with a finger tip cracking open (usually in the winter.) However, I know other people who are in various professions whos fingers crack open during the winter as well.
Here is what I do when I develop a crack. I use a cuticle trimmer and trim back the thick callous type skin. Callous is dead skin and it will never grow back together. All it will do is get thicker and separate more, which prevents the growing layer of skin underneath it from healing properly. Then, I apply a few layers of liquid bandage (New Skin brand or something similar.) I then wear a finger cot (new one with each client) until it heals. The finger cot will help keep oils or anything else from getting in the crack. Clients wont even know you have it on and it will keep you from potentially scratching clients.