Type gem is a term used to describe four different types of gemstones: OPAL, JADE, ONYX, and AGATE. OPAL is a gemstone composed of hydrated amorphous silica, which gives it a unique play of color. JADE is a gemstone composed of two minerals, nephrite and jadeite, and is known for its green color. ONYX is a banded variety of the silicate mineral chalcedony and is usually black or dark brown in color. Lastly, AGATE is a type of quartz that is characterized by its banded patterns and is available in a variety of colors.