1) Cavity in a rock 2) Geode 3) Rock cavity 4) Small cavity in a rock 
Small cavity in a rock filled or lined with crystals/minerals that are different than the host rock.
Vugs (also spelled vugh) are small to medium-sized cavities inside rock that may be formed through a variety of processes. Most commonly cracks and fissures opened by tectonic activity (folding and faulting) are partially filled by quartz, calcite, and other secondary minerals. Open spaces within ancient collapse breccias are another important sou...
small cavity in a rock
1. a cavity in a rock. 2. a small cavern, larger than a pore but too small to contain a person. Typically found in limestone subject to groundwater leaching. 
A cavity in rock; sometimes lines or filled with either amorphous or crystalline material; common in calcareous rocks such as marble or limestone.
a hollow space in a rock where crystals often grow, e.g. Herkimer diamond in a vug
A cavity in rock sometimes lined or filled with either amorphous or crystalline material, common in calcareous rocks such as marble or limestone.
A small cavity in a rock, frequently lined with well-formed crystals. Amethyst commonly forms in these cavities.