User blog comment:Alchimous/Stones of Binding?/@comment-25345260-20130704021414

I'd say they are LARGE gemstones, adding to the rarity of them. They can be used for any large amounts of energy... Say, Tectonic, Ascension shards, destruction of matter to energy, all as examples.

I'm also tired, and don't have time to explain why. But I think being huge gemstones would explain the price and rarity.