Hexagonal growth zoning is the pattern of angular colour or growth bands within a corundum crystal that follows the hexagonal symmetry of the crystal structure. The pattern is one of the most reliable diagnostic features for natural corundum and is used by gemologists to distinguish natural material from flame-fusion (Verneuil) synthetic corundum, which displays a curved growth pattern rather than angular zoning.

Origin of the pattern

Corundum (aluminium oxide, Al2O3) crystallises in the trigonal system, which produces external crystal forms with hexagonal symmetry: six-sided prisms with hexagonal cross-section, and pinacoid or rhombohedral terminations. Growth in nature occurs in pulses, with each pulse depositing a thin layer of crystal that adds to the existing surfaces. When the chemical environment changes between pulses (for example, with the introduction of trace amounts of iron, titanium, chromium, or vanadium), the colour or trace-element composition of the deposited layer differs from the previous layer.

The result is a series of concentric layers within the crystal, each separated by a discontinuity that may be visible under magnification as a faint line of changed colour or transparency. Because the crystal structure is hexagonal, these layers follow the hexagonal symmetry of the crystal. Looking down the c-axis (the threefold rotation axis perpendicular to the basal pinacoid), the zoning appears as a hexagonal pattern. Looking along the a-axis (in the prism plane), the zoning appears as parallel straight lines.

Diagnostic value

The angular hexagonal zoning is one of the most reliable indicators of natural origin in corundum. Verneuil synthetic corundum, produced by flame fusion, shows a characteristic curved growth pattern reflecting the dome-shaped boule from which it grows; the growth bands are curved arcs rather than straight or angular lines. The presence of clear angular hexagonal zoning therefore rules out Verneuil synthetic origin.

Flux-grown synthetic corundum and Czochralski synthetic corundum can produce angular growth zoning that is more difficult to distinguish from natural pattern. In these cases, the zoning is one feature among several that the laboratory considers, and the determination relies on the inclusion suite, trace-element chemistry, and other features in addition to the zoning pattern. For Verneuil distinction, however, hexagonal zoning is generally sufficient on its own.

Trade implications

For trade buyers, the recognition of hexagonal growth zoning is one of the standard gemological skills taught in identification courses. Examination is generally performed with a 10x to 30x magnification, immersion in methylene iodide or another high-refractive-index liquid to reduce surface reflection, and oblique fibre-optic lighting. The zoning is often most visible when the stone is viewed off-axis from the visible facets, looking in along the prism direction of the original crystal.

Hexagonal zoning is also a quality factor in finished stones. Visible angular colour zoning, while diagnostic of natural origin, can detract from the appearance of a faceted stone if the cutter has not oriented the rough to minimise the visible bands. Skilled cutters select the orientation of the table and culet to spread any colour zoning evenly across the face of the stone, but in heavily zoned material this is not always possible. Stones with prominent visible angular colour zoning trade at a discount to evenly coloured material of the same body colour and clarity.