Updated.
How's this?
Disease (Ex): A creature struck by a silver silatic must succeed on a DC X Fortitude save or acquire a disease called argyria. The disease has an onset of 1 day. The victim's skin and eyes become sensistive to sunlight. The victim is dazzled in bright sunlight or within the radius of a daylight spell, and suffers a -2 penalty to Concentration checks in these light conditions. The save DC is Constitution-based.
We still need...
Speed: x ft. (x squares)
Gold is 30 ft., the others are 15 ft.
Feats: 3
We gave gold Cleave, Improved Initiative, Improved Sunder, Power Attack
We gave platinum Ability Focus (acid), Improved Initiative, Improved Sunder, Power Attack
We gave iron Improved Initiative, Improved Sunder, Power Attack
A typical specimen weighs about x pounds. Big Mac's findings upthread indicate silver has a specific gravity of about half of gold, so we could just halve the gold's weight, giving us 150 pounds. But, that makes it less than iron at 200 pounds, which supposedly weighs less than silver.
We currently have...
Iron = 200 pounds
Gold = 300 pounds
Platinum = 600 pounds
We could revise to...
Iron = 150 pounds
Silver = 200 pounds
Gold = 400 pounds
Platinum = 800 pounds
...or just make silver 150 and drop iron to 125.