Does any one have an idea what a diamond has for his soak score? I'm not an expert in all that stuff, and if i do know that diamonds are the element the most (adjective => resistant? solid?... i don't know), I'm not sure it qualify for soak score.
Here it's about an automata of a dog, being made of diamond.
I'd go with whatever you wanted it to be!
Diamond: hardest natural material (one form of carbon nanotubes/variant buckyball is slightly stronger). Can be 'cleaved' to give facets, otherwise needs another diamond to scratch the surface.
You could give it total resistance to piercing (and slashing) type weapons, with a ridiculous bonus to crushing (eg +100), but also allow a roll for its soak - just to check for a 'botch' (I know that soak isn't rolled in 5th edition). This could represent the weapon hitting an unseen fracture plane just right, therefore no soak at all and the whole thing breaks into pieces.
It would have little resistance vs fire at all -it burns the same way that coal does - a bit of concentrated heat and 'pouf' it's just carbon dioxide vapour. Pretty much immune to cold, air, water, but not lightning - that would start it burning.
Overall, I wouldn't make an automaton out of diamond. Thieves would regard it as too valuable (mundanes); and it risks catastrophic fracturing. A nice metal alloy (high steel? 'adamantium' (get the recipe from a dwarven smith - there's a story there) or other fancy material of your choice).
Gilarius (the materials chemist)
In fact, the magus has a magical focus in gemstones so he uses a spell to do his material. Imaginem disguise prevents any thief. Besides, diamond as priceless gems give the best material shape possible for an automaton, which is also important .
I will consider giving the automaton a ward against fire and lightning.
Diamond is not quite so easy to burn as that - it's very strongly bonded, has a very high thermal conductivity so it's hard to get local hot spots and tends to have very low surface area relative to volume as smooth unscratched crystals. It will burn if you heat it red hot with a blow torch and drop it in liquid oxygen but this was unknown in medieval times. I'm not sure if it's brittleness was known either - faceted gems came later and Pliny recommended testing diamonds by hitting them really hard with a hammer! Anyway, if you can magically shape diamond you could make a diamond composite that is far less brittle.
However, this does not mean that diamond offers perfect protection. Firstly, it can only protect what it covers. Armour requires gaps and joints for sight, breath, cooling, removability and articulation and even an unliving construct will need the last of these. Secondly, diamond is light and rigid so it does not absorb impacts, it transmits them. Sufficient battering may destroy non-diamond components within despite never penetrating. Thirdly, even diamond is just stuff - the right spell and... poof!
How about soak +20 for the construct, +17 for a diamond based coat of plates/ full harness but 5 less vs impact.
Soak isn't rolled in combat, but Soak is rolled for non-combat damage, including damage caused by spells - see "Injuries", ArM5 page 181.