Can I return a fake gem bought in Sri Lanka?

You can actually return a fake gem and insist on a refund, assuming you bought it from a shop that is NGJA-licensed, plus you have clear proof of fraud. But trying to recover your money from unlicensed street vendors is, in practice, basically impossible.

If an approved store sells you a synthetic stone while saying it is natural, then you can submit a formal complaint to the National Gem and Jewellery Authority or the Tourist Police. The NGJA can revoke their trading permit, and they may even make them issue a refund. Still, if you purchased the stone from a beach boy or a tuk-tuk driver’s friend, then your cash is effectively gone forever.