Vitreous from Latin 'Vitrum' Meaning 'Glass'
Vitreous which is the most common type of lustre seen in minerals and gemstones describes the way light interacts with the surface of the stone. Stones with a vitreous lustre are often described as being 'glassy' but the term is generally frowned upon. Although glass is produced from natural minerals it's a man-made product so is not a mineral in its own right. Natural minerals are crystalline solids meaning they're made up of crystals. Glass is an amorphous solid meaning it does not have a crystalline structure.
The word 'vitreous' comes from the Latin word 'vitrum' meaning 'glass'.
Obsidian is natural volcanic glass which forms as a result of lava cooling so quickly there was little or no time for crystals to grow. Naturally occurring solids that are not crystalline are correctly known as mineraloids. A mineraloid is an amorphous solid.
Stones described as having a vitreous lustre will reflect light to some degree. All transparent and most translucent gemstones exhibit a vitreous lustre. The word implies the appearance or physical properties of the mineral or gemstone are 'glass-like'.