In the episode 'serious steven' the CGs went to the strawberry battlefield where a upside down pyrimid is floating.
Inside is the pyramid has mural portraying the gem battle plus gives you a glipmse into some of its deadly rooms.
Afterwards the gems began to try to escape the pyramid but notice its a type maze/trap. then after realizing they travel to the power source where the rooms of the pyramid are controlled. There you can see the obelisk statue, a rectangular statue with floating pyramids on the top and bottom.
one of the diamonds
the face shown has a frown and below it there is a gemstone that is white and diamond shaped. then garnet throws steven to the statue and presses his gem against the statue. the head pyramid starts to turn until the smiling one is shown which trigures the diamond to fall out.
obelisk smiling
2 frowning
gem
%
Another is in the episode 'horror club' when steven h=goes to the haunted lighthouse where there is a corrupted gem hidden. the gem has powers likr the white one ,in which it can control or power an entire building. It possesses the lighthouse and haunts steven and the others. the gem acted this way towards lars mostly but also 'ate' sadie in a human like mouth.
The creepy Lighthouse Gem Monster that possessed the lighthouse.
Then steven collects the gem and bubbled it, it looks alot like a blue diamond.
look
Maybe the smiling face of the obelisk is supposed to be rose as she is a nice and compassionate gem whereas the other faces represent white,blue,and yellow. these are what i think are blue and white diamond after they were trapped by rose on earth. they both controlled entire buildings. though white diamond seemed to be the only reason the pyramid was even standing since it exploded after it was removed. and maybe thats why jasper and peridot only mention one diamond even though there are obviously more than just one.
DISCLAIMER this is my theory even though im about 90% sure