What describes a flat break in a gemstone caused by included minerals?

Parting refers specifically to the tendency of certain minerals to break along specific planes of weakness due to internal factors, such as the presence of included minerals or structural characteristics within the gemstone. This phenomenon occurs when the mineral's crystal structure has planes along which bonds are weaker, allowing the stone to break more easily in those planes.

In contrast, cleavage describes the internal arrangement of atoms and the inherent tendencies of a crystal to split along defined planes. While related to how a gem might break, it specifically refers to the natural planes of weakness determined by the crystal lattice, independent of included minerals. Facets describe the flat surfaces that are created by cutting gemstones, and twinning planes are specific configurations related to how a crystal develops its structure, typically in a distinctly symmetrical manner.

In summary, the concept of parting encompasses the effects of internal flaws, such as included minerals, that promote a flat break in gemstones, making it the most fitting description in this scenario.