If you want to save some hassle and confusion, try using the Ectotherm Tortoise Mineral Block. It has a soft powdery texture and contain calcium carbonate (like the cuttlebone) plus some other minerals and vitamins (Vitamin A, Vitamin D3, Vitamin E, Vitamin C, Folic Acid, Thiamine, Riboflavin, Niacin, Pyridoxine, Vitamin B12, Biotin, Panthoric Acid) which the cuttlebone lacks.
I feel this a much better alternative, and have had great success with my tortoises gnawing on them. Here is a link to where you can find this and other great tortoise product...
http://ectotherm.com/ect/mineralblock.htm
Enjoy!