A colour sector — also termed sectoral colour zoning — is a discrete region of colour within a gemstone that corresponds to a specific crystallographic growth sector rather than to a concentric growth layer. During crystal growth, different crystal faces (and the pyramidal or prismatic sectors that grow behind them) may incorporate trace elements at markedly different rates. The result is a stone whose colour is distributed in angular, wedge-shaped or hourglass-shaped domains that mirror the internal geometry of the crystal, rather than in the parallel bands associated with conventional colour zoning.

Mechanism of Formation

Crystal growth proceeds face by face: each set of equivalent faces — for example, the basal pinacoid, the first-order prism faces, or the dipyramidal faces of a hexagonal crystal — constitutes a growth sector. The partition coefficient for a given trace element (the ratio of its concentration in the growing crystal to its concentration in the surrounding melt or solution) differs from one set of faces to another. In corundum, for instance, iron and titanium — the principal chromophores responsible for blue colour in sapphire — are taken up more readily by certain pyramidal sectors than by adjacent prism sectors. The boundary between two sectors is therefore a compositional discontinuity that manifests as a sharp colour contrast, even though no interruption in growth occurred.

Appearance in Sapphire and Corundum

Sapphire is the gemstone in which colour sectors are most widely documented and gemmologically significant. Viewed down the c-axis (the optic axis of the hexagonal crystal), sectoral zoning may produce a six-rayed or three-rayed angular pattern. Viewed perpendicular to the c-axis — the orientation most commonly encountered in a faceted stone — the sectors appear as wedge-shaped or triangular colour concentrations, often with one pyramidal sector noticeably darker or more saturated than the flanking prism sectors. In some Kashmir and Ceylon sapphires, the interplay of lightly and more deeply coloured sectors contributes to the characteristic velvety or sleepy appearance prized by connoisseurs, as light scatters differently through regions of varying chromophore concentration.

Occurrence in Other Species

Colour sectors are by no means confined to corundum. They are well documented in:

• Tourmaline — particularly in elbaite, where the trigonal symmetry of the crystal produces three-fold sectoral patterns. The famous hourglass tourmalines of certain localities display a dark triangular sector alternating with paler sectors in cross-section.
• Vesuvianite (idocrase) — sectoral zoning is a characteristic feature, often producing brown and green sectors in the same crystal.
• Synthetic corundum and flux-grown synthetics — the absence of natural sectoral zoning, or the presence of anomalous curved sector boundaries, can be a diagnostic indicator when distinguishing natural from synthetic stones.

Gemmological Significance

Colour sectors are of practical importance in several contexts. First, they serve as a natural-origin indicator: the specific angular geometry of sectoral zoning in corundum is consistent with natural crystallisation conditions and is difficult to replicate in flame-fusion (Verneuil) synthetics, which instead display curved growth striations. Flux-grown and hydrothermal synthetics may show sector-like features, but the boundary character and associated inclusions differ from those of natural stones, and experienced gemmologists use a combination of microscopy and spectroscopy to distinguish them. Second, the position of a colour sector relative to the cut stone affects face-up colour: a cutter who orients a dark pyramidal sector toward the culet of a sapphire can concentrate colour in the table, making a lighter rough appear more saturated in the finished gem. Third, in strongly pleochroic species such as tourmaline, sectoral zoning interacts with pleochroism to produce complex colour effects that vary with viewing direction in ways that simple concentric zoning would not.

Observation Techniques

Colour sectors are most clearly revealed by immersion microscopy, in which the stone is submerged in a liquid of matching or near-matching refractive index to suppress surface reflections. Fibre-optic or darkfield illumination enhances the contrast between sectors. In sapphires, examination perpendicular to the table — looking through the pavilion — often reveals the angular sector boundaries most clearly. Photoluminescence imaging and micro-X-ray fluorescence mapping have been used in laboratory settings to map the distribution of chromophore elements across sectors with high spatial resolution.

Further Reading

• GIA Gems & Gemology — Colour Zoning in Corundum
• GIA Gems & Gemology (general archive)