Reflection scattering is the diffuse return of light caused by reflection from numerous small internal surfaces — fine silk inclusions, fluid pockets, sub-microscopic particles, or structural discontinuities — distributed through the body of a gemstone. The cumulative effect of countless individual reflection events is a milky, hazy, or sleepy appearance that reduces transparency without altering the body colour. The phenomenon is also termed diffuse internal reflection in older literature.

How it differs from absorption and from Tyndall scattering

Reflection scattering is a physical phenomenon rather than a chemical one. It does not depend on the spectroscopic absorption that produces body colour; the stone can show pure red, pure blue, or any other hue while simultaneously appearing milky because internal scattering is degrading transparency. The phenomenon is related to but distinct from Tyndall scattering, which depends on particle size relative to wavelength and tends to favour shorter wavelengths. Reflection scattering involves reflection from discrete internal surfaces and is generally less wavelength-selective.

Common occurrences

Fine rutile silk in sapphire, particularly in unheated Kashmir and Burmese material, produces the velvety bloom and milky undertone that the trade values for its softening effect on saturation. Heat treatment partially or wholly dissolves the silk and removes the milky appearance, which is one of the reasons heat is so commonly applied to corundum. Microscopic fluid inclusions in emerald produce reflection scattering that contributes to the cloudy jardin character of much Colombian material. Sub-microscopic particles in chalcedony cause the diffuse internal scattering responsible for its waxy translucency.

In the trade

Reflection scattering is not necessarily a defect. The velvet of a Kashmir sapphire is a textbook case of trade-valued scattering, where the very same internal silk that reduces measured transparency provides the soft, deep, velvety body for which the variety is famous. In other materials — clean blue sapphire from Sri Lanka, clean ruby from Mozambique — the same scattering would be a flaw. The grader must understand both the physics and the market context to know whether to call the appearance a virtue or a fault.

Further reading

• GIA Gem Encyclopedia — silk and inclusions in corundum
• GIA Gems & Gemology — Kashmir sapphire and silk
• Lotus Gemology library — corundum inclusions and origin
• International Gem Society — internal scattering and transparency