There's a table of material values somewhere in the help documentation - IIRC this came up before, and gem armor may actually be Gem encrusted wood, from the raws ...
looking at the raws:
[REACTION:GEM_BPLATE]
[NAME:make a gem breastplate]
[BUILDING:GEMWORKSHOP:CUSTOM_T]
[REAGENT:A:3:SMALLGEM:NO_SUBTYPE:INORGANIC:NONE]
[PRODUCT:100:1:ARMOR:ITEM_ARMOR_BREASTPLATE:PLANT_MAT:
GEMTIP:WOOD]
[SKILL:CUTGEM]
[PLANT:
GEMTIP] - to simulate gem arrow points
[NAME:arrowwood][NAME_PLURAL:arrowwood][ADJ:sharp]
[USE_MATERIAL_TEMPLATE:STRUCTURAL:STRUCTURAL_PLANT_TEMPLATE]
[BASIC_MAT:LOCAL_PLANT_MAT:STRUCTURAL]
[USE_MATERIAL_TEMPLATE:WOOD:
GEMTIP_TEMPLATE]
[MATERIAL_VALUE:2][VALUE:1]
[SOLID_DENSITY:5500]
[MATERIAL_TEMPLATE:
GEMTIP_TEMPLATE]
[MATERIAL_VALUE:1]
[SPEC_HEAT:800]
[IGNITE_POINT:NONE]
[MELTING_POINT:11500]
[BOILING_POINT:14000]
[SOLID_DENSITY:7850][IMPACT_YIELD:1000000][IMPACT_FRACTURE:1000000]
[COMPRESSIVE_YIELD:1000000][COMPRESSIVE_FRACTURE:1000000]
[SHEAR_YIELD:100000]
[SHEAR_FRACTURE:100000]
[SHEAR_STRAIN_AT_YIELD:130]
[ABSORPTION:0]
[ITEMS_AMMO]
[WOOD]
NOTE:
NO ELASTICITY DEFINED! (defaults to 0 (lower is better!))
ok... mixed numbers here - Compressive yield and fracture is between IRON and STEEL, but the defaulted elasticity number is equal to adamanite. Shear numbers are twice as good as steel. - so depending on the attack, either a bit better or a bit worse than steal, but better than IRON.
(see
http://dwarffortresswiki.org/index.php/DF2012:Metal#Weapon_.26_Armor_Quality)
Just another note:
[REACTION:HELM_UPGRADE_ONE]
[NAME:Add plates to helm]
[BUILDING:ARMORY:CUSTOM_H][BUILDING:MAGMA_ARMORY:CUSTOM_H]
[REAGENT:A:1:HELM:ITEM_HELM_HELM:NONE:NONE]
[REAGENT:plate:1:TOY:ITEM_TOY_ARMORPLATE:NONE:NONE]
[PRODUCT:100:1:HELM:ITEM_HELM_PLATE:GET_MATERIAL_FROM_REAGENT:A:NONE]
[SKILL:FORGE_ARMOR][FUEL]
there is no material type on the plate reagent - this means that any armor can be plated with any material ... Copper plated adamantine would produce plate adamantine.