c. 1476 CE
A polishing wheel makes symmetrical facets possible.
Read the full sourced page: Diamonds Get Their Facets
Myth: undefined
Reality: The first faceted diamonds, the point cut of the later 14th century, just polished the natural crystal. Symmetrical facets came with van Bercken's scaif around 1476, and the modern round brilliant only in the 20th century.
The Gem Tray is another reference site by the same author.