From the last episode,"Lars Head", Steven along with two Off-Colors gems wall up the "entrance" with a big rock; episode can be watched here: https://drive.google.com/drive/folders/0B4kAJwTQ6mdKeGlFWHFuSDlPS3M.
Before start, I'll make notice that no one in SU has an official height, the crewniverse are enought lazy to create a height chart take sthe freedom to draw the characters as they want; so the height here will be based in this episode and statements from previous one: Steven has grown issues since the 8 years old (iirc), so using this chart, I'll place him in 135 cm, If you are wondering why I put him so small, see the right pic, he's around half of Lars height, and there has been episodes where he seems smaller.
- Steven’s height = 43.33 px + 28.635 px + 120.814 px = 193.28 px = 135 cm
- Rock diameter = 581.32 px = 406 cm ==> Radius = 203 cm
- Rock height = 570.46 px = 398.45 cm
- Volume = 34389393.82 cc
- Mass (granite) = 2.9*34389393.82/1000 = 99729.242 kg
- Minimal force to push (hard rock on soft rock)) = 0.65*99729.242 = 64824 kg
I known that Rutile and Rhodonite "helped" Steven a little, but Steven made almost all the work just don't known how many; to lowball, I'll assume that Steven contributed with a 50%, so final result would be: 32412 kg (Class 50)