I had my cats all “chipped” last Fall. The vet showed me the chips. The actual chip is in a rice size glass tube with melted ends. It’s a piece of glass and I’ve never heard of glass causing cancer. I can’t think of anything that would be more inert.
Considering all the things a cat or dog will eat while out in the yard, I’m amazed they make out as well as they do.
I’ve seen my cats eat crickets, mice, birds, butterflies, worms, snails, and grass. Who’s to say that the mouse didn’t eat something poisonous before the cat got him. We has a 14 year old cat die 6 years ago. Afterward we found out that he ate a pinecone.