Definition

An indented twin is a clarity feature in faceted diamonds, more rarely in coloured stones, in which a crystal twinning plane intersects the surface of the stone, producing a visible line where the two twinned regions of the crystal meet. The twin itself is a feature of the original crystal growth, formed when crystallisation conditions caused two parts of the crystal to grow in mirror-image orientation across a shared plane. The cutter, in faceting the stone, has cut across the twinning plane, exposing it as a surface feature.

Identification

Indented twins are visible under 10x magnification as straight lines crossing one or more facets, often with a slight indentation along the line, hence the name. The line is typically straight or nearly so over short distances, reflecting the planar geometry of the twinning. Where the line crosses a facet boundary, it may show a small step, since the polishing of the two facets has left the twinning plane at slightly different depths.

The line may have associated optical effects, including slight birefringence visible under crossed polarisers, since the two twinned regions have different crystallographic orientations. In well-developed twins, internal graining lines parallel to the twinning plane may also be visible.

Causes

Twinning in diamond is most commonly the macle twin, in which two diamonds grow in mirror-image orientation across an octahedral plane, producing a flattened triangular crystal called a macle. Macles are common in alluvial and primary diamond deposits and constitute a meaningful fraction of rough production. When a macle is cut into a faceted stone, the original twinning plane is preserved within the cut crystal, and depending on cutting orientation it may emerge at the surface as an indented twin.

In coloured stones, twinning is variable across species. Quartz commonly twins, with Brazil twinning and Dauphin twinning being the principal forms, both of which can produce indented twin features when cut and polished. Corundum, particularly ruby and sapphire, can show polysynthetic twinning that produces fine parallel lines visible as indented features.

Clarity grading

GIA grades indented twins based on visibility and extent. Minor indented twins may have minimal grading impact; significant indented twins, particularly those crossing the table or visible from the crown, can reduce the grade by one or more steps. The feature is plotted on the GIA grading report and described in the comments section.

Implications for cutting and durability

The twinning plane is, in some cases, a plane of weakness in the crystal. Cutters working with twinned rough are aware of the orientation of the twinning plane and prefer to orient the finished stone such that the twinning plane is not aligned with stress-bearing facets. The girdle and pavilion main facets are particularly sensitive, since impact stress at the girdle can cause crack propagation along a twinning plane in some cases.

For the working setter, an indented twin in the girdle or near the girdle is a feature to note before working the stone. Bezel and partial-bezel settings reduce the stress on these regions and are preferred over four-prong settings for stones with significant girdle-region twinning.