An interesting thread. I don’t recall ever seeing a cleavage in a sapphire. Cleavage stated in the books for sapphire is, “None”. I agree with Syd above … more likely a fracture that has fingerprint healed. You do see discoid or halo fractures from heat treatment though. The only other thing that comes to my mind, without seeing the stone of course, would be the possibility of a twinning plane. Would you care to post a photo so we can see what you are seeing?