What is this?

Two photos: One of them shows growth just before it came off, the other shows immediately after it came off.   Earlier this year I noticed one of my three cats had a skin growth appearing on her back. As it grew it formed a sort of ball. You can see in the photo it looks like layers of skin growing in concentric circles. Touching the growth didn't seem to cause her any discomfort, just as we were about to take her to the vet, the growth detached. I was stroking her gently and it just came away as my hand brushed it. It left a perfectly circular little scab. I cleaned it and kept an eye on her and the scab disappeared incredibly quickly. Out of curiosity, rather than just throw the detached ball away I decided to cut it in half. I was half dreading finding a tick or something inside it, but it was indeed just layers and layers of skin. Cat is absolutely fine, she's healthy and happy, but I've always wondered, what was this? I've search online a lot and have never seen any other photos which look anything like it. To satisfy my curiosity, has anyone seen this before or does anyone know what caused it?   Many thanks! :)

Hello - I am not sure of this to be honest.  It looks like some kind of keratinised structure or cyst, but i cannot remember having seen these come away so cleanly. I wonder whether you still have the lump, as your vet might either have a good guess or send it away for you for testing.  It is unreliable to guess this sort of thing from the outside appearance and we would always recommend that if your vet cannot easy explain it, you consider having a sample checked / sent away for testing.