How many letters does the top answer for “the art of engraving on precious stones” have?

The most common answer is glyptics (8 letters).

Crossword clue: the art of engraving on precious stones

1 answer found in Letterora's clue database (Wikidata facts, Open Trivia DB, WordNet and Webster definitions — all licensed sources).

glyptics 8 letters wn-def

the art of engraving on precious stones

the art or process of engraving on precious stones. 80% similar → glyptography