Adding Magic and Physical Rainbow facet jewel versions has been suggested quite a few times now and I'm sure mods are sick of reading it. The problems are two - there is no % physical skill damage and Pierce would be way too OP for both magic and phys builds. Not to mention Rainbow jewels roll -3-5. There's a reason no -magic resist items exist and so few - phys resist items exist. Lot's the latter have also been nerfed - I remember soul drainers rolling -5to10 and now they are -2to4. Crossbows were also -10 at first. A jewel with phys pierce would also be exploited by non-caster phys builds and make them OP.
So the new jewel is: Yin-Yang facet. It's 2 times rarer than Rainbow facet.
It rolls either magic or phys version.
Magic version:
100% chance to cast Bone Nova when you level up OR 100% chance to cast Blessed Hammer when you die
18-26 magic damage
2 to 4% magic skill damage
In addition to this Rare magic skill dmg jewels should probably be nerfed a bit.
The physical version uses a new mechanic called Deadly Cast. I got this from discord when people were discussing whether the current Deadly strike bug should be kept for skills - it feels great for lots of people, but 4xLo runes in weapon would be OP. The Deadly Cast is a crit which works only for your physical spells. But unlike Crit and DS it's capped at 50%.
Physical version:
100% chance to cast War Cry when you level up OR 100% chance to cast Tornado when you die
2 to 4% chance for Deadly cast
2 to 4% deadly cast multiplier
% extra Multiplier works just like %Deadly Strike Multiplier. Deadly cast default damage multiplier should be 100%. Now you can reach up to 50% chance to deal 150+% damage with your physical caster spells with 13 jewels in gear.
Too late for this season, maybe we'll see it in later season, but I love it.