Geode (noun)

1. A hollow, spherical or oval-shaped rock with a cavity lined with crystals or mineral deposits.
2. A spherical or oval-shaped rock that is hollow inside and lined with crystals or mineral deposits.

Origin:

From french géode, from greek geodes "earthy", from ge "earth" + -odes "like".

Examples:

1. The geode was filled with sparkling crystals.
2. The geode had a rough, dull exterior that hid the beautiful crystals inside.
3. The geode was used as a decorative item in the garden.
4. The geode is a type of rock that is often used in lapidary work.
5. The geode was formed when a hollow cavity in a rock filled with mineral-rich fluids that then crystallized.