a smooth surface (as of a bone or cut gemstone)
"The jeweler carefully polished each facet of the diamond to ensure it caught the light perfectly."
Any one of the flat surfaces cut into a gem.
"The jeweler carefully polished each facet of the diamond to make it sparkle brilliantly under the light."
To cut a facet into a gemstone.
"The skilled artisan carefully polished each facet of the rough diamond until it sparkled brilliantly under the light."
The word facet comes from the French term facette, which originally meant a small flat surface or side of a gemstone.