For the final installation of the histories, I’ll be considering the last biocrystal, ianthite.
This is the biocrystal of communication, or rather of sending signals — whether they be visual, audial, or by other means (the exact means would be determined by cerulite attached).
The best use I can think of for this would be as a kind of wire between different biocrystals, allowing for circuitry and some programming for coordinating biocrystals. Though the first uses that people would find for it would be for signals, like flashlights or smoke signals — not exactly smoke signals, that is, but probably beacons of some kind that could be used in about the same way as smoke signals — or alarms.
Potentially, the medium of the signal could be modified and concentrated into a weapon, or for uses other than communication. Ianthite that emits light could be modified to shoot a laser; ianthite that emits sound could shoot rays; perhaps even ianthite could be made into guns or other projectile weapons.