Mayco Astro Gem Glaze - AS510 - White Opal - 4 Ounce Jar Astro Gems are an underglaze-based textural glaze that contains tiny crystals. When fired, these crystals provide jewel-like spots on the matte surface of the glaze. Astro Gems can be applied to both green-ware and bisque. As surfaces may be difficult to properly clean if in contact with food, we recommend Astro Gems be used on decorative ware. Astro Gems are certified AP non-toxic. Fire to shelf cone 06. Made in USA.