A diapson crystal is a crystal used to draw down energy from the sun. Diapson crystals are not magic, but a form of new technology discovered during the time of the Druid Walker Boh. They are used to power airships in the Shannara series, greatly changing the nature of travel and commerce in the Four Lands.
Later, diapson crystals are used to created deadly weapons, from large fire launchers mounted on airships to wall-mounted and handheld "flash rips."
The first prototype weapons were so deadly that Grianne Ohmsford, the Ard Rhys of the Third Druid Order, negotiated a treaty between all parties in the Four Lands forbidding the use of diapson crystal weapons. This was her final act before giving herself over to the Tanequil. However, centuries later this treaty is often ignored, at least unofficially.