Let's see, it's got 4 more HD than a 2e iron golem, 1 die better damage, 4 better THAC0, and much better AC (-6 vs. 3).
So I'm thinking...
3d10 damage (vs. the iron's 2d10)
+3 better Str modifier than iron (which is Str 33)
+30 natural armor (8 higher than iron, equivalent to adamantine golem, but not as good as ironwyrm)
Str 39, Dex 9, Con —, Int —, Wis 11, Cha 1
Look OK?
Through the door is a tremendous statue standing on a pedestal of stone.
The diamond golem is one of Zagig’s final creations, a behemoth he manufactured to protect his most valuable treasures.