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About Minerals

Geodes

Step into a rock museum or collector's shop and you will immediately light on the geodes. Resistance is futile. These are among the most enchanting and puzzling of Earth's creations.

There are conflicting views about how geodes form but most geologists agree that they begin as bubbles in underwater limestone sediments or in a lava flow. Occasionally they form around the body of a sea creature on the ocean floor. Over time the bubble is cast in a hard shell of silica, and water containing dissolved material is trapped inside. The precipitate that results is composed of a variety of minerals, usually quartz or calcite, bit also aragonite. Many thousands of years later, weathering lifts the hard little silica orb away from the surrounding rock.

The Exterior of the sphere appears dull and pitted, which is no doubt why the Greeks called them geodes, meaning "Earth-like". However, on the inside, they can look heavenly. The precipitate forms a lining of inward projecting crystals and, since different materials harden at varying temperatures, the material forms layers.

Sometimes a cavity may be completely filled by layers. Purists refer to these geodes as nodules. Crack open a nodule and instead of a hollow crystalline core, you might find halos of brilliantly hued agate, produced where chalcedony-laden water precipitated within the shell.

The best locations to look are in deserts in the Western US, especially Arizona, Utah and Nevada. BC is well known for it's own brand of geodes known as Thunder-eggs.

Geodes come in many sizes, from marble sized to as large as 18 inches in diameter. Once you uncover a geode, tap it firmly as you would test a watermelon for ripeness and listen for the telltale hollow sound. Then, with a well-placed blow, split the rock to unlock its secrets.

via Port Moody Newsletter and The Rock Vein, March/April, 2001.