It is the strongest modifier for both fire resistance and damage, but the damage modifier can only appear on magic Jewels (not Rare).
Ruby Jewels may have either fire resist or +damage, there are two different "Ruby" prefixes for jewels that merely use the same label.
Can appear on: Magic Jewels
Fire Resist +10-11% (Small)
Fire Resist +13-15% (Large)
Fire Resist +26-30% (Grand)