Agate is banded or patterned chalcedony in the quartz family. Bands, plumes, moss-like patterns, translucency, and polish matter more than a single ideal color.
What to notice in agate
Begin with the material rather than a romantic name. Look at color in more than one light, then study transparency, inclusions, polish, symmetry, and face-up brightness. In a faceted stone, check for a pale window through the center and dark extinction that stays fixed as the stone moves. In a cabochon or carving, judge the surface, outline, pattern, and any optical effect from several angles.
Carat weight is not a measurement of visible size. Compare the millimeter dimensions and depth as well. A deep stone can carry hidden weight; a shallow one may spread well but lose color or brilliance. Photographs should show the individual piece, and a slow video can reveal movement that a still frame misses.
Wear, care, and disclosure
Agate has a Mohs hardness of approximately 6.5 to 7, but hardness only describes resistance to scratching. Cleavage, brittleness, inclusions, thin edges, and setting style also affect how safely a gem can be worn. Dyeing and heat treatment are common in some commercial agates, so unusually vivid or uniform color deserves disclosure.
Ask what is known about identity and treatment, what has been independently tested, and which details remain undocumented. Geographic origin should not be guaranteed from appearance alone. A laboratory report can be sensible when identity, treatment, origin, or price would materially change the decision.
Gems Jewelry may not currently offer this material for sale. This page remains a practical reference for collectors, jewelers, students, and anyone who enjoys looking closely. For a wider inspection method, read the natural gemstone buying guide and the treatment disclosure guide.