From the manufacturer:
Add reflective shine to your handwriting and illustrations. Metallic has a distinctive reflective sheen, similar to the bright colors of polished hot rods. This shiny, opaque Gelly Roll® writes on both light and dark papers as well as glossy and matte surfaces. Additionally, Sakura�s archival quality gel ink ensures that your work (and the metallic sheen) will last for years. So put pen to paper and create an artistic, polished mark that�s uniquely your own.