Whilst the individual items themselves give a single stat atop their meager +2 Power to a single domain, the strangely statted Prism Jewel only supplies +2 Power to 6 very specific domains (Nerifian, Mercalan, Sylphid, Isesipian, Huggessoan, and Mercalan). It doesn't carry over anything special, and doesn't have much going for it. Unlike Prism Jewel, the Chimera Bands actually do see some, if infrequent use, due to its incredible stat increase across the board, as well as retaining a weakness to make it more interesting of an item.
While I do want to see the Prism Jewel get use, I think giving it the Plus treatment will net in it being a more interesting item, if only as a gimmick; what I suggest is an item for those that want to run more than just a few elements in their spell pool.
Reasonings:Prism Jewel Plus
Materials: Metal / Remains
+1 STR, +1 WIL, +1 SKI, +1 CEL, +1 RES, +1 VIT
+10% Power to all Spells, regardless of Domain.
-10% FP cost to all Spells, regardless of Domain.
Does not halve your Power when casting a Spell of a different Domain type to your Tome's Domain type.
-The +1 to 6 different, thinly spread stats will prevent people from using this item all of the time, and raw stats alone will not win you a fight.
-The +10% Power is mostly more of a modern effect in line with the tome fans, though the six specific domains from the original Prism Jewel didn't make much sense on their own, so I'd rather opt for it to increase to all domains- it's only a marginal increase, but larger than two.
-The -10% FP cost is mostly an added bonus for upgrading this frankly bad item all the way, which this will do quite literally nothing to some spells and decrease the FP cost by 5 at most, unless you use something with more than 50 base FP costs, which is extremely uncommon.
-The final effect is more to encourage multi-elemental casting, since by the wording of the "About Tomes" section leads me to believe power is halved when your Chapter type is different from the spell's domain, which frankly happens often anyway.