Oh! I forgot to mention, quills have been used too...
One thing about the bone is that it can be filed back into shape... plastic would just create plastic dust. So even if it doesn't keep its point after a while, it could be restored.
Looking through wikipedia, more historically used plectra include, coconut shell and mesrab, which is made of metal wire. There are all sorts of plectra made from various gemstones too:
https://elmntpicks.com/ I suppose the major qualification is that they are a hard and polishable enough material that they will not abrade at the strings, but will pluck. Harpsichords have been made with leather plectra, and I've used little pieces of birch bark too (but not with too much success--it was quite quiet).