say I have a {Serum Powder} In hand and i also have a {Gemstone Caverns} In hand. Can I put then caverns in then use the powder for 6 new cards?
From the rulings of {Gemstone Caverns}:
6/1/2011 Your "opening hand" is the hand of cards you decide to start the game with after taking any mulligans.
So, you would first mulligan your hand with {Serum Powder}, then put {Gemstone Caverns} on the battlefield if it is in your opening hand after mulligans.
Ok thanks