‘% damage on block’ appears to be physical damage of an (unstated) base value around 85, independent of your weapon or shield. You can increase it with armour penetration and physical damage boosts.
Gems only affect (and consider) the item they are socketed into. A gem with 40% enchantment power if it’s the only elemental gem …
doesn’t boost other items
doesn’t care about gems in other items, even if they are elemental gems
always works if it’s the only gem in a magical/plagued item
also boosts the other three gems in a common item unless they are elemental gems
Better descriptions are coming eventually, possibly next patch. There are a few others, like lifesteal values being only a third of what is displayed, that will be fixed too.
Haven’t done extended testing myself beyond verifying that weapon damage has no direct scaling effect, but there is a guide on this forum that outlines this behaviour: