What is a geode?

A geode is specifically a spherical mineral-lined cavity that is often found within rocks. This unique geological formation occurs when mineral-rich water enters a cavity in rock, leading to the deposition of minerals in a crystalline form as the water evaporates. Over time, this process creates a hollow center that is lined with crystals, often of quartz or calcite, which can be visually stunning when the geode is cut open.

The other options offer definitions that do not accurately describe a geode. A refers to a mineral that changes form, which is unrelated to the geological structure of a geode. C describes a gemstone with a distinctive color pattern but does not capture the essence of a geode. D mentions a gemstone that has gone through heating, which is not relevant to the creation or definition of a geode. Thus, the identification of a geode as a mineral-lined cavity aligns with geological and mineralogical principles.