Increased elemental damage vs increased fire damage on ruby jewels

Hi, I am trying to figure out how to scale my gas grenade explosion damage on my witch hunter. When looking for the affixes on different jewels, I got puzzled by the prefixes "X% increased fire damage" and "X% increased elemental damage" both of which can role on ruby jewels. Does the latter scale my explode damage the same way increased fire damage does or am I missing something? Would a ruby jewel with both modifiers be something I should be looking for?
Any explanation would be highly appreciated.

Bump. I'm curious about this as well.