OK, I am obviously not talking about Hawksbill tortoiseshell - so illegal it almost hurts to LOOK at a pick. So we have created a thousand analogues, including things like synthetic plastics (delrin, ultex, etc.) and polymerized natural proteins (galalith et al).
But there are green turtles which are entirely unthreatened, and have comparable shells. There are horns from a dozen animals which are made of keratin. There are HORSE HOOVES that get routinely trimmed. Why are these not being used to replace actual tortoiseshell, despite being chemically closest to it?
Synthetic materials are largely superior in the boutique space at this point if you care about performance characteristics