Filocite
Physical manifestations of the weave, frequently purple, but blue in its densest form, hence the city's name.
The depths of Bluetrench are particularly rich of the substance, hence the city's and the name of The Mages of Bluetrench.
Each Shardling of Bluetrench is granted a Filocite crystal to be used as a spellbook / personal repository. Moreover, the CrystalWrights branch of the College is dedicated to the study of Filocite and its use in various crafts and disciplines.
#alex :
Filocite is typically understood to be a naturally-forming mineral, but as is the case in Rakir's crater-coast, it can solidify over time after ore is exposed to a high concentration of magic all at once, like Trinitite. The reason behind Bluetrench's rich Filocite deposits could be because it sits at a Weave Node (also known by more nature-y folk as a Leyline), a place where the Weave is particularly tangled and dense.
On Filocite Spellshards
The way it is overwritten and unlocked does tell Riga that the crack might be partially responsible for the informational jumble. Since these spellshards store information inside of their physical form, tearing a chunk off may very well mean some information is lost.
It would be comparable to slashing off pieces of a book's pages, for example