What does twinning refer to in gemology?

Twinning in gemology refers to a phenomenon where the crystal structure of a gemstone exhibits a distinctive arrangement caused by a change in its growth direction. This often results in multiple intergrown or mirrored lattice structures within a single crystal. It can significantly influence the characteristics of the gem, including its appearance and optical properties. Twinning typically occurs under certain conditions during the growth of the crystal when it experiences varying environmental factors such as pressure or temperature, leading to this unique orientation.

The other options describe different processes or features but do not accurately define twinning. For instance, grinding gems into uniform shapes relates to the cutting and shaping process, whereas the replication of gemstone patterns through polishing pertains more to surface finishing techniques. Lastly, layering of different minerals refers to stratification rather than the specific internal structure changes that take place in twinning. Thus, the correct answer highlights the pivotal role of crystal direction alteration in the context of gemology.