Agates vs. Jaspers: A Field Guide

Both agate and jasper are varieties of chalcedony, which is microcrystalline quartz. Chemically, they are identical (SiO2). The difference lies in their structure and impurities, which dictate how they look and transmit light.

The simplest field test is light. Agate is translucent—if you hold it up to the sun or a flashlight, light passes through it (at least through the edges). Jasper is opaque; it is packed with so many impurities (like iron oxides) that light cannot pass through it, even on thin edges.

Agates typically form in cavities (vesicles) in volcanic rock layer by layer, creating concentric banding. Jaspers are often sedimentary or metamorphic in origin and tend to have massive, spotted, or brecciated patterns rather than concentric bands.

Famous agates include Lake Superior Agates, Botswana Agates, and Moss Agates (which have inclusions that look like plants). Famous jaspers include Red Jasper, Ocean Jasper, and Picture Jasper (which looks like landscapes).