Agates bring natural beauty into your home. This set includes four polished agate slab coasters in the color purple. Agate is a naturally formed semi-precious gemstone that includes the minerals chalcedony and quartz. Each slab of agate is unique in structure and crystal habit, so one slab may have more layers of bands and another may have more visible crystals. All agates have crystalline structures, but some are cryptocrystalline, meaning you cannot see it with the naked eye, but you can under a microscope. Each agate coaster measures at least 3.25 inches in length and 5mm in thickness.