What does the term "allochromatic" refer to?

The term "allochromatic" refers to a gem that derives its color from external factors rather than its internal crystal structure. This means that allochromatic stones are typically colored by impurities or trace elements that are present in the crystal lattice.

For instance, when certain trace elements like chromium or iron are present in a mineral, they can impart distinct colors to the gemstone. This differs from "idiochromatic" gemstones, which are inherently colored due to the elements present in their basic chemical composition. Hence, understanding that allochromatic refers specifically to color imparted through external influences underlines why this choice is correct.

In contrast, the other options address different concepts; multiple colors in different lighting refers to phenomena such as pleochroism or color change rather than to allochromatic properties, while a completely colorless gem is simply colorless and not considered allochromatic. A gem that changes color when warmed implies a temperature-dependent reaction, which does not relate to the allochromatic definition focused on colors from external sources.