I have had my store bought (wild caught?) male Russian Tort for 4 weeks now and wondering if this chipped keratin will ever grow back. He was like this when I got him.
If it's a big enough injury, yes, new keratin does grow back UNDER the white, dead bone. And because the new shell is growing and that dead bone is NOT growing, the volume of the new shell eventually makes that old, smaller dead piece pop off. But, in your tortoise's case, I doubt there's going to be any new growth underneath that dead spot. It's just not big enough.