The mask first appeared in Symphony of the Night, where it gives DEF +2 and an INT boost of +4. This is the same amount of defense that the other mask in the game, the Ballroom Mask, gives, although the latter doesn't give an INT boost.
It's called Jade Mask in Lament of Innocence, where it can be used to cause candles to be destroyed by simply walking into them, making the collection of hearts much easier. This is available at Rinaldo's shop for 999 gold after the player beats the game, saves, and loads the cleared save file.
It also appeared in Curse of Darkness, where it gives both STR and CON +5 and DEF +2, but it's also cursed and reduces LCK by 5. It can be created by combining Jade with Sun Tears. The Stone Mask can be immediately evolved to a Face Guard, which only gives +3 DEF and therefore has no use at this point of the game. The Stone Mask ultimately can evolve into a Masakado's Helm, which is a much improved version of the Stone Mask (STR +15, CON +10, DEF +10, LCK -10), or to a Pumpkin Mask, which can evolve to the Pumpkin Mace needed to gain the Pumpkininnocent devil. You should wait until you have all the materials necessary to evolve the Stone Mask to the Masakado's Mask before evolving it into anything else, since you will likely have no reason to equip any of the intermediate items.
Only Jonathan can equip the Stone Mask in Portrait of Ruin. It gives him DEF +3 and LCK +1. It can be compared to the Ballroom Masque, which instead of increasing Luck increases INT by 1 (and also gives a DEF +4). In this game, it's the Skull Mask the one which is cursed, which increases DEF greatly by 13, but at the cost of -3 to INT, MND and LCK.
Funerary jade masks originate from diverse prehispanic cultures; mainly from those that were situated from Middle to South America. While most in-game descriptions state they were used by shamans in ancient rituals, real life masks were instead placed on the corpses of important people, such as kings, during their entombment.
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