Not sure what Opal, Topaz, or Jade's parodies are.

Sugilite is The Son of Man.

Aventurine is The Cheat with the Ace of Clubs.

Obsidian is The Portrait of Adele Bloch-Bauer I.

Opal's is based off one of Van Gogh's straw hat self-portraits.
I swear I've seen the basis for Jade's piece but it escapes me, probably something involving Andromeda or Eve.
Sapphire(?) seems to be parodying some depiction of a griffon from an illuminated manuscript/medieval bestiary.