Inclusion.

An inclusion is anything that found itself caught inside a crystal while the host was growing. It can be another mineral, a tiny pocket of fluid left over from the growing solution, a bubble of gas, or even a thin slice of the surrounding rock. Inclusions are common, often beautiful, and frequently the most interesting thing about a piece.

Some of the best-known crystal varieties are named for the inclusions they carry. Rutilated quartz holds golden needles of rutile (titanium dioxide). Tourmalinated quartz holds black tourmaline rods. Lodolite or "garden quartz" carries cloudy ghosts of chlorite, feldspar, and other minerals that look like landscapes inside the stone. Hematite, mica, dumortierite, and even small water bubbles all show up regularly.

Inclusions tell a geological story: what else was present in the fluid when the crystal grew, what temperature and pressure conditions were like, and how the surroundings changed over time. In modern crystal practice they are often read as adding character or carrying their own meanings: rutile for clarity of thought, tourmaline for protection layered into amplification. A clean stone is not better than an included one. They are different kinds of beautiful.