Seems mildly costly. Elements contains surprisingly few Element based cards, Nightfall and Holy Light being the main exceptions. Perhaps the cost is a bit high?
Perhaps, but Sturdy is a bit of a strong ability, seeing as it makes creatures twice has hard to kill. Were you to stick it on a Chimera with 25 hitpoints, it could take up to 50 points of damage before dying, even more if you had something healing it. It also makes creatures with growth far stronger. It could even be used to counter Flooding/Inundation, as it would change the opponents creatures to Earth, thus making them vulnerable to Flooding/Inundation.