I have been working on sorting out the order in which different metals add elemental damage. It appears that they each do it in a specific order. I have tested it enough that I am pretty comfortable that this is how it works.  Please let me know if you see anything different:
Copper:  20 Energy  and then 
10 PoisonAgapite:  30 Cold  and then 
20 EnergyValorite:  20 Cold , 
20 Energy , 
10 Fire , 
10 Poison Verite:   20 Energy  then 
40 Poison The single category enhance metals are as follows:
Bronze: 40 Fire  Shadow:  20 Cold  Of course instead of testing it myself I could have just found this information on Stratics, but where is the fun in that.  
http://vboards.stratics.com/showthread.php?t=62865&highlight=enhance Basra on stratics mentions a rumor that there have sometimes been exceptions to the standard order (Cold, Energy, Fire, Poison) and states that EA has denied that this is the case.  I can verify that it does (or at least has) happen.  I imbued a 90% dagger some time ago with copper and it became 100% poison.