I use tortoise shell picks on mandolin, but not on guitar. They're stiffer per their thickness than plastic picks, and can give you a little extra "snap" in your picking. As for how they're made, the shell of tortoises is split (it builds up in layers, I think, and can be separated that same way) and then the thin layers (NOT the whole shell) is shaped by cutting and grinding into picks. Their cachet and the ban on selling any "new" ones makes them pricy little things, probably more expensive per gram than most anything short of a diamond (or maybe cocaine). I got a half dozen in 1963 in Des Moines, IA, and still have most of 'em (one split during playing, so I still use the broken piece). They are definitely individual as far as tone -- no two sound or feel the same).
That answer enough questions?