As fascinating as the myriad variety of sparkling colors found in spinel gemstones can be, the rough crystal formations from which the polished gems emerge hold their own allure. Collectors covet these mathematical wonders of atomic structure for both their scientific and aesthetic value. As a member of the cubic crystal system, spinel crystals exhibit the same physical properties in all directions throughout the crystal. Spinels typically form in an octahedral shape, which somewhat resembles two pyramids stuck together at their bases.
While typically well-formed, if a crystal is subjected to differential stress or pressure conditions during the growth process, it can cause the creation of two or more intergrown symmetrical crystals. This phenomenon is known as crystal twinning, a process in which two separate crystals share crystal lattice points in a variety of precise conformations.

There are three primary conditions under which twinned crystals develop. The first, known as growth twins, occur as the result of a change in the lattice structure during formation because of an impinging larger ion. The second condition, known as transformation twins, occurs because the crystal system somehow cools during formation. As a result, instability occurs within the crystal system, and the structure must transform into a more stable form. In the third and final condition, gliding twins occur when the crystal is subjected to stress after formation (GeologyIn; Types of Twinning in Minerals).

Spinel Law and Contact Twins

Twinned crystals are scientifically classified by a series of twin laws which are specific to individual mineral systems, making them useful as a diagnostic tool in mineral identification. For example, Spinel Law is named for the most common habit of the mineral spinel. In this scenario, the crystals are contact twins possessing a twin plane parallel to the octahedral face. The plane behaves as a mirror plane not parallel to the other mirror planes, but still producing identical left and right sides. In other words, the mirror plane divides the crystal in half, with the two octahedral faces parallel to the plane forming equilateral triangles. Each point of the triangles is doubled across the twin plane with an indentation between them. The crystal looks like it has trigonal symmetry, but the indentations are the tell-tale indicator that the crystal is a twin (Rock & Gem; Crystal Twinning, July 2020). If the angle is 180 degrees, the crystals will grow away from each other, but if the angle is less than 180 degrees the twin will have a noticeable bend, frequently forming what are known as dove-tail, fish-tail, and chevron shaped twins.

Contact twins can also occur as a more complex formation of multiple twins. If their surfaces are parallel, they are called polysynthetic twins, and if they are not parallel, they are known as cyclical twins.

Penetration Twins

In penetration twins, like the Star of David crystal formation so prized among spinel specimens, the twinned crystals penetrate each other. These penetration twins develop when a crystal begins to form, but an offset of ions along the axis of the developing crystal allows a second crystal to grow inside it. The secondary crystal grows at a different angle, based on the directional course of the first crystal axis. The second crystal inhabits a portion of the first, its corners protruding from the center of both faces of the host crystal. These twin sets, when well-formed, can sometimes produce crosses and other complex structures. Penetration twins are delineated by a twin axis and have a variable composition surface separating two individual crystals.