Have you checked the books for your particular species? That is the first place to start, and also the Tortoise Trust and other decent websites.
My own thoughts are that it could be either a need for more calcium (so the cuttlefish idea is a good one) or, less probable in my view, a habit similar to some creatures that swallow stones to either aid in crushing up food (birds) or as a means of countering buoyancy (crocs). (Almost certainly not the latter!). Since the body of a tortoise lacks the slightly expansive capabilities of most other vertebrates, I'd be a bit worried if I thought it were taking in stones.
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