Agate

Agate is one of the oldest ornamental stones in continuous use, carved into seals and beads for millennia, and it remains popular today for the same reason it always was: pattern. Where amethyst or citrine grow as large, clear single crystals, agate is chalcedony — the cryptocrystalline, fibrous form of quartz — and it forms in layers. Silica-bearing groundwater seeps into a cavity, usually a gas bubble in cooling volcanic rock, and deposits silica in concentric bands over time. Each band can differ slightly in porosity, trace-element content, and color, which is why a polished agate slice shows rings, waves, or eye-like patterns rather than a uniform hue.

Species and variety. Agate is a variety of chalcedony, itself a form of quartz (SiO2), distinguished from macrocrystalline quartz varieties like amethyst and citrine by its fibrous, aggregate microstructure rather than a single large crystal lattice. Within the chalcedony family, agate is defined specifically by its banding — plain, uniformly colored chalcedony is called by other names (chrysoprase if green, carnelian if orange-red), and banded material with straight black-and-white layers is more specifically called onyx.

Appearance and color range. Natural agate ranges through grays, browns, and muted reds, often with white or bluish bands, though commercial agate is very frequently dyed — bright blue, green, or purple “agate” in mass-market jewelry is almost always natural gray or white banded chalcedony that has been artificially colored, since the material’s porosity along band boundaries takes dye readily. This is worth knowing before buying: undyed agate is the more muted, geological-looking stone; vividly saturated agate slices sold as décor or jewelry are dyed.

Hardness and care. At roughly 6.5–7 on the Mohs scale, agate is durable enough for everyday rings, bracelets, and pendants, resisting scratches from most household contact. It is not as tough as it is hard, however — a sharp knock can still chip or fracture it, so it is best kept away from other jewelry in a pouch or lined box, and cleaned with mild soap and water rather than ultrasonic cleaners, which can worsen existing internal fractures.

Where it is found. Major sources include Brazil, India, Uruguay, Botswana, Madagascar, and the United States, with Brazilian and Uruguayan agate nodules — often lined with amethyst inside — particularly well known in the trade.

Birthstone role. Agate’s birthstone status is historical rather than current: it appears in pre-20th-century monthly lists for May and June, but drops out of every major modern system (US 1912, UK 2013, US current). It survives, though, as the traditional tropical-zodiac stone for Gemini — one instance of the broader pattern where a sign’s stone traces back to the historical monthly list for the month that sign begins in.