Luck

Known Effects:
Summoning: +5% yield

Speculated Effects:
Chest/Map/Bottle: Increases chance of item generation
Exploration: Bonus 25%?
(After about 100, 1 turn explores I've never seen it go over 150% average or under 50% average with or without Star of luck)

Star of Luck from different sources stack.

Sources:
Voting
Wish
God Favor