Now that I think of it, perhaps I should have said where the idea ultimately came from. Although I had purchased the Fire & Ice back in 2004 or 2005, I never really read/studied it until the last month. Was just too busy with work, college, new home, new lady friend, later switching jobs, etc.
While I was reading the Fire & Ice, I was also re-reading The Silmarillion. Instead of gems that captured the True Light, I decided that an Arcane Elementalist Forger, with the help of Master Elementalists, created the Elemental Gems. As Fëanor said, "...there [are] some deeds that [one] may accomplish but once only... ...never again shall [any] make their like..."
So... Basically, the Elemental Gems would be Artifacts, if not Legendary Artifacts. They are one-of-a-kind gems, never to be repeated. Although the image I hashed up in the first post does not show any kind of size, I planned on the gems being about 2½ cm major axis diameter and a minor axis diameter half of that (1¼ cm). That gives them a total volume of about 32.75 cm³. Thus, they would weigh about 2300 g (2.3 kg, or about 5 pounds). And that is for each gem. All 10 would weigh about 23kg or about 50 pounds. Remember, I listed the density as 20× that of diamond which is about 3.5 g/cm³. Thus, milvar would be about 70 g/cm³.
I might rethink that density. However, remember milvar does not exist naturally. It is literally an artificially created elemental substance through supernatural means, much like the silma in the silmarils. In this case elemental means like helium is an element, not as fire is an element. Milvar is completely colorless and transparent, or in technical terms, it is lucid.
The +50 may seem a bit high, but look at some of the bonuses for the Artifacts and Legendary Artifacts listed in the Treasure Companion and the Creatures & Treasures books. With just a casual glimpse through the Artifacts, I constantly saw +25, +30, +40, +50, +60 bonuses all over. Although keyed, I even saw one bonus of +75! Wow!
As VladD mentioned, a history for each gem would be required. Or, I could just say that the "elves" felt they were treasures so valuable, like the silmarils, and have kept them locked away. Perhaps kept them locked away for the time when they would be needed. Perhaps a prophecy of "the one who can wield all the elements" without penalty or harm.
I also like the idea of using the penalties when trying wield more than one gem at a time. Page 23 of Fire & Ice gives a listing of how elements are compliment, neutral, source, or scourge of each other. Perhaps make the gems follow that. Example, an Air Elementalist could actually have a -50 modifier for controlling Light through the Light gem since Light is the Scourge of Air. This will take some work.
Keep the feedback coming because there may be others who may wish to use this idea.
rmfr