The term 'hard, glassy mineral' refers to the gemstone GARNET, which is a 6-letter word. GARNET is a group of minerals that come in a variety of colors, including red, orange, yellow, green, blue, purple, brown, black, pink, and colorless. It is composed of two or more elements, and is known for its hardness and luster. GARNET is often used as a gemstone in jewelry, and is also used in industrial applications such as abrasives and water filtration.