Identification

How to Tell if a Rock Is an Agate

Agates are banded rocks made mostly of chalcedony, a variety of quartz. Michigan's environment agency (EGLE) explains that they form when gas bubbles in lava leave holes that silica-rich solutions slowly fill, layer by layer. The softer host rock weathers away and the harder agates are left behind.

Field clues

• Translucence. Hold the stone up to the sun or a flashlight. Agates let some light through. Banded rocks that are not translucent, like banded jasper or laminated sandstone, are not agates.
• Waxy luster. EGLE describes a dull, waxy shine on unpolished agates.
• Pockmarked surface. Many have small pits on the outside.
• Iron staining. Minnesota DNR says Lake Superior agates usually show rust-red or yellow iron-oxide staining.
• "Peeled" texture. Where an agate has broken along its bands, the surface can look partly peeled, like a banana skin (Minnesota DNR).
• Wet them. Minnesota DNR suggests looking after rain or wetting stones, since colors look richer wet.

Even experts can't always tell from the outside. Bands are often hidden until a stone is cut or tumbled.

Common agate types

Minnesota DNR describes fortification agates (bands that connect like fort walls), water-level or parallel-banded agates, and the rarer eye agates with round "eyes" on the surface.

Gear

Collecting on beaches and public land has rules. See our guide to collecting rules.

Sources: Michigan EGLE; Minnesota DNR, Minnesota's State Gem.