Cross-Polarized Filters / Strain Patterns

Introduction

Diamond is an isotropic crystal — in theory, it should not affect the polarisation of light passing through it. But in practice, internal strain from lattice distortion causes a phenomenon called anomalous birefringence, where the diamond bends polarised light in ways it theoretically should not. Viewing a diamond between two crossed polarisers reveals these strain patterns as coloured interference patterns against a dark background.

For natural versus lab-grown screening, the type of strain pattern — or its absence — provides a quick and informative clue about a diamond's origin.

How Cross-Polarized Observation Works

The setup is straightforward:

1. Place the diamond between two polarising filters oriented at 90° to each other (crossed polarisers)
2. Illuminate from below
3. Observe the diamond from above

In an ideal isotropic crystal, crossed polarisers would produce a completely dark field — no light would pass through. But strain within the diamond lattice creates localised birefringence that allows some light through, producing coloured interference patterns in the areas where strain is present.

The pattern of these coloured areas reveals the history and nature of the strain — and by extension, the conditions under which the diamond formed.

Natural Diamond Strain Patterns

Natural diamonds have spent billions of years in the Earth's mantle, subjected to enormous pressures and tectonic forces. This extended exposure to stress causes plastic deformation — the crystal lattice is permanently distorted in localised zones.

Under cross-polarized light, this deformation produces characteristic patterns:

Tatami pattern. The most recognisable natural strain pattern — a cross-hatch of intersecting lines resembling the woven surface of a Japanese tatami mat. These lines follow the slip planes of the diamond lattice and record the history of directional stress the crystal experienced during its time in the mantle.

Irregular, complex patterns. Natural strain is rarely uniform. The interference colours vary across the stone, following the complex stress history of geological residence. The pattern is typically asymmetric and distributed unevenly.

High-colour, high-order interference. Because natural deformation can be substantial, the birefringence patterns often show vivid, high-order interference colours — blues, greens, oranges, pinks — rather than simple dark-and-light.

Lab-Grown Diamond Strain Patterns

CVD Diamonds

CVD diamonds grow in a stress-free environment — a low-pressure plasma chamber with no tectonic forces, no compression history, and no plastic deformation. The result under cross-polarized light is typically:

• No strain pattern — the field remains dark, with minimal or no anomalous birefringence
• Some CVD diamonds may show faint, localised strain near the seed interface or at growth boundaries, but this is qualitatively different from the pervasive tatami pattern of natural diamonds

HPHT Diamonds

HPHT diamonds grow under extreme pressure, so some strain can be present. However, the pattern differs from natural deformation:

• Strain tends to follow the cuboctahedral growth sector boundaries rather than the irregular slip-plane patterns of mantle deformation
• The overall strain level is typically lower than in naturally deformed diamonds
• The pattern, when visible, is more regular and geometric than the chaotic tatami of natural stones

Diagnostic Value

Cross-polarized observation is a useful screening indicator:

Limitations

Cross-polarized screening has important limitations:

• Not all natural diamonds show strong strain. Some natural diamonds — particularly high-quality Type IIa stones — experienced minimal plastic deformation and may show little or no tatami pattern.
• Some lab-grown diamonds show some strain. Particularly HPHT-grown stones, which experienced real pressure during growth.
• The method is qualitative. It relies on the observer's experience in recognising pattern types, and borderline cases require further testing.
• It cannot identify the growth method. Cross-polarized light distinguishes "strained" from "unstrained" but does not specify CVD versus HPHT or natural Type Ia versus Type IIa.

For these reasons, cross-polarized observation is best used as part of the broader screening workflow — a quick check that adds evidence to the overall assessment rather than a standalone determination.

Equipment

The equipment required is minimal and inexpensive:

• Two polarising filters (available from gemological suppliers)
• A light source (transmitted light from below)
• A gemological microscope or loupe for observation

Many gemological microscopes include polarising filters as standard accessories. The observation takes seconds and is completely non-destructive.

Recommended tools from Arete:

• Gemetrix StrainView — a dedicated strain-imaging instrument for visualising anomalous birefringence patterns in diamonds
• Classical Table Polariscope — a standard bench polariscope for cross-polarized observation
• Polarisation Filters for Stereo Microscope — a filter set that adds cross-polarized capability to existing stereo microscopes

Frequently Asked Questions

Can I use cross-polarized light to definitively identify a lab-grown diamond?

No. Cross-polarized observation is a screening tool, not a definitive identifier. It provides evidence that, combined with other methods (UV, spectroscopy, DiamondView), builds toward a conclusion.

What if a natural diamond shows no strain?

This happens, particularly in high-quality Type IIa natural diamonds. The absence of strain does not confirm lab-grown origin — it means the diamond did not experience significant plastic deformation. Further testing is needed.

What does the tatami pattern look like?

A grid-like pattern of intersecting coloured lines, resembling woven matting, visible against a dark background when viewing the diamond between crossed polarisers. The colours are interference colours and may include blues, greens, oranges, and pinks.

Is this technique difficult to learn?

The basic observation is simple — anyone can place a diamond between two polarisers and look. Interpreting the patterns requires experience. A gemologist with training in strain analysis can distinguish natural tatami from HPHT sector patterns relatively quickly, but borderline cases require practice.

Summary

Cross-polarized light reveals internal strain patterns in diamonds. Natural diamonds typically show tatami or cross-hatch interference patterns from billions of years of mantle stress. CVD diamonds usually show no strain, and HPHT diamonds may show geometric sector-boundary patterns. The technique is quick, inexpensive, and non-destructive — a practical screening step that adds useful evidence to the identification workflow. Its limitation is that it is not definitive alone: some naturals lack strain, and some lab-grown stones show some. It works best as one tool among several.