Rock Types.docx

Geodes are spherical to subspherical rock structures with an internal cavity lined with mineral materials. They have a durable outer wall that is more resistant to weathering than the surrounding bedrock. This allows the geode to survive intact when the surrounding bedrock weathers away. The mineral lining the cavity is often a scintillating druse of tiny quartz crystals underlain by multiple bands of translucent gray and white agate. Many are lined with more spectacular treasures. Rich purple amethyst, perfect white calcite crystals, and colorful banded agate are other common linings. Rare geodes can be filled with beautiful blue gem silica, pink rhodochrosite, spectacular opal with vivid play-of-color or other rare materials. Geodes range in size from under one centimeter to several meters in length. From the outside most geodes look like common rocks, but when they are opened the sight can be breathtaking.

Our Favorite Book About Geodes! Geodes: Nature's Treasures by Brad L. Cross and June Culp Zeitner.

The Public’s Love Affair With Geodes Most geologists enjoy geodes. However, the general public has a love affair with them. They are delighted and amazed that an uninteresting rock can contain a beautiful cluster of gemmy crystals, or a colorful lining of banded agate, or both of those in the same cavity. Broken open, or sawn and polished, people who have never taken a geology course buy thousands of tons of geodes each year. They buy them because they enjoy them. They love tiny geodes as jewelry, sawn and polished geodes as bookends, and spectacular amethyst geodes as items of home or office decor. In several parts of the world, geode localities have spawned profitable industries that collect them, prepare them for market, and ship them to destinations where they are purchased as items of science, natural art and enjoyment. Brazil, Uruguay, Mexico, and Namibia are four examples of countries where geodes have become a local industry. Geodes sell rapidly at gem and mineral shows, science museums, rock shops, art galleries and shops that feature international or natural gifts. When geode-opening demonstrations are given at rock and mineral shows, they always draw a crowd -- and that crowd usually produces enthusiastic cheers and gasps when a nice geode is opened. There is something special about a beautiful treasure hiding in a rock that looks absolutely ordinary from the outside.

Geodes in basalt: An outcrop of a basalt flow exposing several open amethyst geodes. This photo illustrates how geodes can occur in large numbers within a single rock unit. Image copyright iStockphoto / yio.

Geologic Occurrence and Formation Geodes are not found randomly here and there. Instead they are usually found in large numbers in areas where the rocks have formed in a special geochemical environment. Most geodes localities are in A) stratified volcanic deposits such as basalts and tuffs; or B) stratified sedimentary carbonate deposits such as limestones and dolomites. A diversity of other environments yield a small number of geodes. Geodes form in many different ways, and there are a number of valid theories about their formation. The intent of this article is not to provide a single or a comprehensive coverage of the many ways that geodes might form.

The Story in the Rock: A spectacular sawn and polished geode specimen with multiple layers of colorful agate and a crystal-filled central cavity. Each colored band represents an episode of agate formation and a change in the composition of the groundwater that delivered mineral material into the geode. Image copyright iStockphoto / WojciechMT.

Volcanic Geodes The most widely known and sought-after geodes are those that formed in areas of volcanic activity. Voids in basaltic lava flows often are infilled with agate, quartz, opal and other material delivered by hydrothermal water or groundwater. Some voids are spaces occupied by gases that failed to escape the lava flow before its surface crusted over. Where does all of the gas come from? Some magmas contain a lot of dissolved gas. They can be several percent dissolved gas on the basis of weight. (Think about that - several percent gas by weight!) When these magmas ascend to the surface, the gas expands in proportion to the pressure reduction. When the magma erupts as a lava flow, so much gas is released that not all of it is able to escape. Some of that gas can be trapped in the lava to produce a large cavity when the lava solidifies.