What term is used for atoms in a gem that are not part of its essential chemical composition?

The correct term for atoms in a gem that are not part of its essential chemical composition is trace elements. Trace elements refer to minor constituents that can be present in very small amounts within a gemstone. These elements can significantly influence the color, clarity, and overall appearance of the gem without being part of the primary structure of the mineral.

For example, the presence of chromium in trace amounts can give emeralds their distinctive green color, while the presence of iron can affect the hues of sapphires. Trace elements often enhance the uniqueness of colored gemstones, contributing to their varying properties and value.

While inclusions might refer to other materials or imperfections within a gemstone, they are not necessarily atoms that exist as part of the gem's chemical structure. Defects generally indicate irregularities in the crystal lattice but do not specifically address the presence of additional atoms. Admixtures can imply a mixture of different substances but is not as specifically related to the concept of trace elements as it applies to gemology.