Geodes: symbols of inner beauty

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Oct 01, 2009

"YOU CAN'T ALWAYS JUDGE A BOOK BY ITS COVER."


Photo 1: Geode with an outer surface of chalcedony.
During the past two decades, we have analyzed more than a half-million uroliths submitted to the Minnesota Urolith Center. Experience has taught us that we cannot reliably judge their mineral composition by their external appearance.

Although evaluation of the outer surface of urinary stones often provides useful information, one must also consider what is beneath the surface in order to obtain reliable diagnostic data. In like fashion, the importance of examining what lies within a patient is the cornerstone of the discipline of internal medicine.

What are geodes?


Photo 2: Cross section of the geode described in Photo 1.
With this principle of looking beyond the surface, consider the examination of another type of stone, the geode. The word "geode" is derived from the Greek word "geoides" meaning earthlike. Geodes typically are spherical, hollow inside and vary from less than an inch to more than a foot in diameter. They have been found throughout the Earth.

At first glance, geodes often look like ordinary stones that blend into the landscape (Photo 1). Their outer layer, often composed of bland-looking chalcedony, gives no indication of what is concealed within. But if you break them open and examine what is inside, often you will find that their subdued surface conceals eye-catching aggregates of inward-projecting, sparkling and colorful crystals (Photo 2).

Although there are many types of geodes, geologists often categorize them into two groups. One group of geodes are formed in igneous rock, particularly lava. In this group, gas bubbles in molten volcanic rock formed hollow spheres when they were trapped as the magma cooled. Subsequently, when water supersaturated with a variety of elements (especially silica) seeped into these hollow spaces, crystals formed and grew inward from the outer wall of the cavity.

The other group of geodes formed in sedimentary rock. Initially, decomposing organic materials became mineralized with calcite (calcium carbonate). Next, changes in the chemical composition and pH of the water in the sediment resulted in replacement of the outer margin of the calcite concretion with a gelatinous semi-permeable shell of silica.

Further changes in the composition and pH of the water percolating through the shell of silica resulted in dissolution and removal of the inner core of calcite, leaving a hollow cavity. With time, the peripheral gel of silica dehydrated and crystallized to form an outer layer (or rind) of chalcedony (microcrystalline quartz whose crystals are too small to be seen with the naked eye).

Cracks subsequently developed in this outer shell, allowing water supersaturated with a variety of elements to enter the cavity. Precipitation of these elements resulted in the unconstrained inward growth of crystals, starting at the rind and projecting into the hollow center.

The inner appearance of geodes is dependent on the environmental conditions (temperature, pH, etc.) in which they formed, and the types of elements in the water that percolated through them. Precipitation of minerals such as quartz or amethyst transformed the insides of geodes into dazzling formations of crystals. The outward characteristics of such geodes may not catch your attention, but you can't miss their inner beauty (Photos 1 and 2).

Not all geodes contain attractive formations of crystals. For example, minerals protruding from the rind of geodes containing large quantities of chalcedony may have a bumpy cauliflower-like appearance. Geodes filled with unorganized silt from the surrounding sediment are sometimes referred to as mud balls.