Tassite is the technical name of the solidfied structure of a clathrate. It varies in color to match the elemental or celestial aspect of the parent clathrate. It is quite hard and durable, ranges from perfectly clear to cloudy opaque, and is a very good spell reagent. Pieces of tassite can be used to store magical essence like a battery. A wizard can use such an item to boost the power of a cast spell (using more dice, or larger dice) or can use it to cast a spell already cast into it.
Natural tassite can be found, but it is so rare that it exists as an oddity, never a commodity.
This is important, and the main reason that Tassite is rare and clathrates remain unknown to even the most educated of magi. This naturally occuring tassite generally seeps to the surface usually as an effect of the magic of the clathrate in question, an effect of erosion, or by displacement by groundwater.