The Lightning Gem is an essence gem which guarantees successful evasion of any traps that players trigger, and also improves their Mind attribute. It is a stronger version of the Slick Gem and the Gem of Premonition.
It can be purchased from Zin Bu after reaching Chapter 3.
In-game description: "Named not for its appearance, but for the speed at which its owner acts, this gem sharpens the mind and senses by orders of magnitude. The wearer becomes smarter and quicker, more easily anticipating or evading the unexpected."