Cross-hatched extinction — also called tatami extinction or the tatami pattern — is an optical phenomenon observed in diamond when the stone is examined between crossed polarising filters on a polariscope. Instead of the uniform dark field expected from an isotropic material, certain diamonds display a grid of alternating light and dark rectangular or banded zones whose appearance closely resembles the woven rush matting of traditional Japanese tatami flooring. The pattern is a direct expression of internal mechanical strain, which induces anomalous double refraction (strain birefringence) in a crystal that would otherwise extinguish uniformly. Its detection is a routine step in advanced diamond gemmology, with practical implications for origin determination, type classification, and the separation of natural from laboratory-grown material.

Physical Basis: Why an Isotropic Mineral Shows Birefringence

Diamond crystallises in the cubic system and is, in theory, optically isotropic: it has no inherent double refraction and should extinguish completely and uniformly when placed between crossed polars. In practice, however, many natural diamonds contain residual stress fields arising from their formation and ascent history. These stresses distort the local crystal lattice, breaking the cubic symmetry on a microscopic scale and inducing a small but measurable birefringence that varies in orientation and magnitude from one region of the stone to another. Where adjacent domains of differing strain orientation meet, the polariscope reveals them as contrasting light and dark zones. When these domains are arranged in a roughly orthogonal grid — a geometry that reflects the {100} and {110} slip planes along which plastic deformation preferentially occurs in diamond — the result is the characteristic cross-hatched or tatami pattern.

The retardation values involved are typically very low, often just a few nanometres, yet the polariscope is sensitive enough to render them visible. The pattern is best observed by rotating the stone slowly under crossed polars: the zones shift in relative brightness as the stone rotates, confirming that the effect is genuinely birefringent rather than an artefact of surface reflection or inclusion scatter.

Association with Diamond Type

Cross-hatched extinction is most strongly associated with Type IIa diamonds — stones that contain no detectable nitrogen in any aggregated or single-substitutional form, as assessed by infrared spectroscopy. Type IIa diamonds are relatively rare in nature (estimates from the Gemological Institute of America place them at roughly 1–2 percent of gem diamonds) but are disproportionately represented among the world's largest and most celebrated stones, including the Cullinan, the Koh-i-Noor, and the Golconda-type colourless diamonds historically prized in the Indian trade. The near-absence of nitrogen impurities means that Type IIa stones are often the most chemically pure diamonds known, yet paradoxically they are also among the most plastically deformed: their journey from the deep mantle through kimberlite or lamproite pipes subjects them to extreme pressure gradients, and without the structural reinforcement that nitrogen aggregates provide, the lattice deforms more readily under stress.

Type Ib diamonds (containing isolated nitrogen atoms) and Type Ia diamonds with high concentrations of B-aggregate nitrogen can also exhibit strain birefringence, though the tatami pattern in its most pronounced rectangular grid form is most characteristic of Type IIa material. Type IIb diamonds (boron-bearing, typically blue) may similarly display the pattern, consistent with their shared low-nitrogen character.

Detection: The Polariscope in Practice

The polariscope is the instrument of choice for observing cross-hatched extinction. In its simplest form it consists of two polarising filters oriented at 90° to one another, with a light source below and an observation position above. The diamond is placed on the lower polar and examined from above through the upper polar (the analyser). A standard gemmological polariscope produces a field that is dark when no birefringent material is present; any double refraction within the stone causes light to leak through the analyser, illuminating the affected zones.

For the tatami pattern to be seen to best advantage, the stone should be examined through multiple orientations — face-up, pavilion-up, and through the girdle — because the strain domains are three-dimensional and may not be equally visible from every direction. The pattern is most legible when the stone is rotated slowly: the rectangular zones brighten and darken in a systematic way that distinguishes strain birefringence from the anomalous extinction caused by twinning or by inclusions. In twinned diamonds (particularly macles and certain octahedral forms), the polariscope reveals broad, sweeping sectors rather than the tight rectangular grid of the tatami pattern.

Diagnostic Value: Natural versus Laboratory-Grown Diamond

The presence and character of strain birefringence has become an increasingly important criterion in the separation of natural from synthetic (laboratory-grown) diamonds, particularly as high-pressure, high-temperature (HPHT) and chemical vapour deposition (CVD) production has scaled dramatically since the 2010s.

• Natural Type IIa diamonds characteristically show the cross-hatched tatami pattern, often in a complex, irregular distribution that reflects the stone's geological history of plastic deformation under varying stress conditions over geological time.
• HPHT-grown diamonds are typically produced under more controlled and uniform pressure conditions. They may show strain birefringence, but the pattern tends to be more regular and sector-shaped, following the growth sectors of the synthetic crystal rather than the irregular deformation domains of a natural stone. Some HPHT synthetics show a cross-shaped or hourglass extinction pattern related to their characteristic cubic or cubo-octahedral growth morphology.
• CVD-grown diamonds frequently display a distinctive striated or banded birefringence pattern oriented parallel to the growth direction (typically the {100} plane), which differs markedly from the tatami pattern of natural Type IIa material. Post-growth HPHT annealing of CVD diamonds can partially reduce this striation, but residual strain patterns often remain detectable.

Gemmological laboratories — including the GIA, Gübelin Gem Lab, and the Swiss Gemmological Institute (SSEF) — use polariscope observation of strain birefringence as one element within a multi-technique protocol for diamond origin determination. No single optical feature is conclusive in isolation; the tatami pattern is most powerful when combined with infrared spectroscopy (for type classification), photoluminescence spectroscopy, and ultraviolet fluorescence imaging.

Cross-hatched Extinction in Other Gem Materials

Although the term is most closely associated with diamond, strain birefringence producing grid-like or banded extinction patterns can be observed in other nominally isotropic gem materials under the polariscope. Synthetic spinels and some garnets may show anomalous double refraction under stress, and certain glass imitations exhibit irregular strain patterns introduced during rapid cooling. In these materials the pattern is generally referred to simply as anomalous extinction or strain birefringence rather than tatami extinction, which remains a term specific to the diamond context in standard gemmological usage.

Significance in Valuation and Trade

The observation of cross-hatched extinction on a polariscope does not diminish a diamond's value; if anything, its strong association with Type IIa material — the category that includes many of the finest colourless and fancy-colour diamonds ever recorded — lends it a degree of prestige in the trade. A pronounced tatami pattern in a large, colourless stone is consistent with, though not proof of, a Golconda or otherwise exceptional origin, and major auction houses and dealers familiar with Type IIa material regard it as a positive indicator warranting further laboratory investigation.

Conversely, the absence of a tatami pattern in a stone presented as a natural Type IIa diamond, or the presence of a pattern inconsistent with natural deformation history, may prompt closer scrutiny. As laboratory-grown diamonds have entered the market at scale, the polariscope — one of the least expensive instruments in a gemmologist's toolkit — has regained prominence as a rapid, non-destructive first-pass screening tool precisely because of its ability to reveal these strain signatures.

Further Reading

• GIA Gems & Gemology — CVD Synthetic Diamonds (2012)
• GIA Gems & Gemology — Lab-Grown Diamond Update (2018)
• GIA — Diamond Type Classification