Multicoloured agate is a type of gemstone that is made up of several different colors. It is a variety of the mineral chalcedony, which is composed of quartz and moganite. ONYX is another type of gemstone that is similar to agate, but it is usually black or dark gray in color. ONYX is also composed of quartz and moganite, but it has a higher concentration of iron oxide, which gives it its darker hue.