Therazane seems to be the worst card imaginable. If you get 3/6 stats for 7 mana, the card text better be good. Well, it isn't. All you get to show is usually 1-2 rockbiters, hardly justifies that seven mana you spent on a 3/6. It could be improved if you increased the health by 4 and attack by 1. Or keep it as is and add the effect to summon an earth elemental as a battlecry.
Maybe Therazane would be better as a 6 mana 3/6 with overload (1) or even a 5 mana minion with same overload. Gazlowe is a 3/6 for 6 by way of comparison.