Ok, now as a summary:
Spike Gem:
- increases base damage by % of (weapon base+attribute bonus)
- thereby increases standard attacks
- thereby increases physical rune attacks
- thereby increases elemental standard attacks indirectly (only minor improvement, but still)
- thereby increases elemental rune attacks indirectly (only minor improvement, but still)
Sources: What weapon gem to use - #11 by _Hattori & What weapon gem to use - #8 by ShortCat
Elemental Gem:
- can not be mixed with other elements
- increases elemental damage by % of base damage, not “dealt” dmg, so enemy armour is irrelevant for elemental dmg
- stacks with other identical elemental damage (e.g. enchantments)
- increases physical rune attacks
- not increases elemental rune attacks
Sources: Enchantments and gems & What weapon gem to use - #8 by ShortCat & What weapon gem to use - #7 by anonymous87 & What weapon gem to use - #10 by ShortCat
→ While the spike gem affects basically everything and also stronger and the elemental gems not, the elemental gems still can create more dmg in some situations since they circumvent enemy armour.
Other info:
- damage roll is done per attack and not per hit, if one attack hits multiple enemies, the attack roll is the same for all of those
- elemental dmg is generally less affected by enemy armour/resistance
Sources: What weapon gem to use - #7 by anonymous87 & What weapon gem to use - #8 by ShortCat