What type of twinning occurs due to environmental changes after the gem has formed?

The correct answer identifies cyclic twinning as the type that occurs due to environmental changes after the gem has formed.

Cyclic twinning happens when there are fluctuations in the conditions under which a crystal is formed or exists. These changes can involve variations in temperature, pressure, or chemical environment that can influence a crystal's internal structure post-formation. As a result, the crystal may begin to exhibit different orientations in its twin domains that reflect these changes over time, resulting in a distinctive patterned appearance.

This type of twinning is generally recognized by the regularity and repetition of the twin planes, often giving rise to a specific and observable symmetry in the gem's structure. The distinction of cyclic twinning lies in its direct relationship to environmental conditions that impact the already formed crystal.

Other types of twinning, such as contact, penetration, or growth, differ in their formation processes or conditions. Contact twinning occurs due to the influence of another crystal that presses against it; penetration twinning happens when two crystals interpenetrate in a manner that they share a lattice structure; and growth twinning takes place when a crystal develops in a way that incorporates twinning during the initial crystal formation. Each of these processes operates under different circumstances and does not specifically rely on