Pleochroism (Grade) — Weak, Moderate, or Strong
Pleochroism (Grade) — Weak, Moderate, or Strong
How laboratories report the strength of selective absorption on coloured-stone reports
The pleochroism grade is the qualitative laboratory descriptor for the strength of colour change observed in a pleochroic gemstone when viewed along different crystallographic axes, recorded on coloured-stone reports as weak, moderate, or strong. The grade describes the range and saturation of the colours observed through a dichroscope and is included on identification reports for species where pleochroism is a meaningful identification, quality, or origin factor.
Grading and observation
Strong pleochroism, as in tanzanite and andalusite, shows distinctly different colours along different vibration directions and is readily observed without instruments by rotating the stone in front of a daylight-equivalent light source. Moderate pleochroism, as in sapphire and many tourmalines, shows a clear difference between the dichroic colours but requires a dichroscope or careful comparison for confident observation. Weak pleochroism, as in most quartz and many garnets, is observable only under careful examination and has limited effect on the appearance of a finished stone.
Grading is performed by an experienced gemmologist using a dichroscope and a controlled lighting environment. The grade is consistent within a species but can vary slightly with body-colour saturation; deeply saturated stones tend to show more pronounced pleochroism than pale material of the same species.
Significance for the buyer
For species where pleochroism affects appearance, the grade is a useful piece of information for the buyer. Strong pleochroism in a stone where the cutter has oriented the rough correctly is a positive feature and is often recorded on the report alongside the colour description. For species where pleochroism is weak, the grade is descriptive but not value-bearing.
In the trade
The pleochroism grade is most likely to appear on reports for tanzanite, alexandrite, andalusite, iolite, and certain tourmaline and corundum varieties. The grade complements the colour description and the dichroic-colour pair recorded on the report.