This is a complex concept to understand, so I clariy myself to help others:
Each Legendary Greensteel shard that you make has 3 parts to it:
Focus = this is pos, neg, air, fire, elect, ice = has nothing to do with set bonuses at all
Essence = this is either Material or Etherial = determines the 4 item set bonuses
Gem = this is either Dom (sp), Mat (hp), Esculation (dodge)
If you happen to have 5 items there is a chart that determines what extra effect happen
http://ddowiki.com/page/Legendary_Green_Steel_items
So, when making a leg greensteel item, if you want an hp bonus, ALL your gem parts of the shard must be Material.
If you want a leg greensteel item with sp bonuses, you only need the clear majority of the gem parts to be Dominion.
I have no idea about Esculation.
If you are actually making 4 or more items then you need to worry about whether you essences should be Mat or Etheral.
I think you just need to the clear majority to be Mat if you are seeking Mat, best I remember.
Have no idea about Etheral.