FacetLab is a free open-source WebGL learning simulator from 4Cs.co.za. Players inspect rough crystal, reveal inclusions, plan orientation, choose proportions, polish facets, and generate a finished 4Cs scorecard. It teaches cut, colour, clarity and carat as connected trade-offs instead of static definitions. No pricing, no appraisal, no signup.

Live tool: https://4cs.co.za/diamond-polishing-game/

GitHub: https://github.com/nichesza-max/facetlab