"Meteoric iron or 'sky-iron' (Tib. gnam lcags) is the supreme substance for forging the physical representation of the vajra or other iron weapons, since it has already been tempered by the celestial gods in its passage across the heavens. The indivisibility of form and emptiness is a perfect metaphor for the image of a meteorite or 'stone fallen from the sky', manifesting out of the voidness of space as a shooting star or fireball, and depositing a chunk of fused 'sky iron' on the earth below. Many vajras held by deities as weapons are described as being forged from meteorite iron, and Tibet, with its high altitude, thin atmosphere and desolate landscape, received an abundance of meteorite fragments. Tibetan vajras were often cast from meteorite iron, and as an act of sympathetic magic a piece of the meteoric iron was often returned to its original site."〔Beer, Robert (1999). ''The Encyclopedia of Tibetan Symbols and Motifs'' (Hardcover). Shambhala. ISBN 1-57062-416-X, ISBN 978-1-57062-416-2. Source: () (accessed: Thursday April 15, 2010), p.234〕