Rock of the Month
Jasper vs. Agate
Story by Bob Jones as printed in the December 2013 Rock & Gem Magazine.
It can be difficult to decide whether a rock is jasper or agate because they have many similar characteristics. Both are considered chalcedony, a cryptocrystalline form of silica, with agate tending toward transparency and jasper toward opacity. Both jasper and agate are hard, Mohs 7, and take a good polish. They exhibit similar patterns, including bands, plumes and eyes, and an array of vivid colors caused by impurities.
Untainted chalcedony is milky white and translucent and lacks eye appeal. Jaspers and agates get their color or distinctive patterns from the inclusion of minerals that are trapped inside the structure. The mineral impurities that usually color agate and jasper are iron oxide or manganese oxides.
Since agates and jaspers are mainly silicon dioxide quartz, the coloring agents end up in the chalcedony by inclusion and infusion. Scientists have shown that chalcedony, despite its hardness, is slightly porous. Because of this porosity, solutions containing several forms of manganese oxides and iron oxides-and in rare cases uranium oxides and other metal salts-can seep into an agate. Jasper has a tighter structure, so the impurities do not accumulate in it to the same extent as in agate. This accounts for the less vibrant colors in jasper.
The original silica gel may also contain impurities. These impurities are the secret to the agate’s many colors. Iron oxides cause the reds and yellows, manganese oxides produce violets and blacks; uranium imparts yellow; copper results in red, green and blue; and nickel salts color the chalcedony green.
From agates to jaspers, chalcedony has given creative artists, architects, jewelry artists and rockhounds a colorful and attractively patterned material we are still fortunate to collect and own.
To read the article in its entirety, please read the article in Rock & Gem.