Agate (specifically, likely a geode slice or nodule)

Hardness: 6.5-7 on Mohs scale; Color: Highly variable, often banded in concentric layers or various patterns, can be white, grey, blue, brown, red, green, black; Luster: Waxy to vitreous; Crystal structure: Cryptocrystalline (microscopic quartz crystals); Cleavage: None; Specific Gravity: 2.58-2.64

Agates form primarily within cavities of volcanic or igneous rocks. Silica-rich fluids, often from dissolved groundwater or hydrothermal solutions, seep into these cavities. Over time, successive layers of microscopic quartz crystals are deposited on the walls of the cavity, forming the characteristic banding. The process can take millions of years, and the geological age varies depending on the host rock's formation. They are often found in areas with past volcanic activity.

Agate is widely used in jewelry (cabochons, beads, pendants like the one pictured), ornamental carvings, bookends, coasters, and as decorative stone. Due to its hardness and durability, it has also been used historically for mortar and pestles, and less commonly in precision instruments.

Agate is one of the oldest known gemstones and has been used for ornamentation since ancient times. The patterns and colors within agate are incredibly diverse, leading to many sub-varieties such as moss agate, lace agate, fire agate, etc. Some agates can also form geode interiors, which may contain larger quartz crystals.

In the field, agate is often identified by its characteristic banding, waxy luster when polished, and its hardness (it will scratch glass). It typically occurs as nodules or fillings in amygdaloidal cavities of igneous rocks (especially basalts and rhyolites). Common locations include Brazil, Uruguay, Mexico, the United States (Oregon, Montana, Wyoming, Lake Superior region), Germany, and India. Collectors look for unique patterns, vibrant colors, and well-formed bands. The specimen shown appears to be a polished slice, likely from an agate nodule or geode, showcasing its internal banding.