Bluefire velvet
From elanthipedia
This is a weaving technique where very fine blue and silver metallic threads are used in the fabric's warp to cause blue lightning to run across its surface when the light hits it.
LOOK: This exquisite and luxurious medium weight cloth is a silken weave with metallic threads in shades of bright blue and silver used sparingly in the warp. When the light hits the fabric just right, the metal's reflection causes an illusion resembling Huldah's flaming, blue lightning crackling across its surface.
This material is required to be provided for alterations.