Hyalite is a form of opal with a glassy and clear appearance which exhibits an internal play of colors and has natural inclusions. It is also called Muller's glass, water opal and jalite. Its Mohs hardness is 5.5 to 6 and it has a specific gravity of 2.1. It is an amorphous form of silica (SiO2). Its has a conchoidal fracture, a vitreous luster and a white streak. It is sometimes mistaken for resin opal, since they both look like little globs. It glows bright green under blacklight.
The name Müller's glass derived from the name of its discoverer, Franz-Joseph Müller von Reichenstein