A dial made from or coated in aventurine glass, a material embedded with fine copper or metal particles that catch light like scattered stars against a deep blue or green base.
Aventurine is a type of glass created by suspending fine copper (or occasionally chromium or other metal) particles within molten glass as it cools, so the particles remain suspended and catch light from within rather than sitting on the surface. The effect resembles a night sky, which is why it's most commonly seen on deep blue dials, though green variants exist too.
The material has been used in decorative arts since at least the 17th century, originating in Venetian glassmaking, and made its way into dial-making relatively recently as a way to bring genuine depth and sparkle to a dial without printing or applying a surface pattern.
Fine copper particles suspended within the glass itself as it cools, so they catch light from within the material rather than sitting on its surface.
Blue is the most common and closely associated colour, resembling a night sky, but green aventurine dials also exist.
The glassmaking technique dates back to at least 17th-century Venice, though its use specifically in watch dials is a more recent development.