inclusions, microscopy, gems

Gübelin explains the principles of phase-contrast microscopy and describes equipment adapted to examining inclusions and subtle internal structures in gemstones. Developed from F. Zernike’s work, the method converts differences in the phase of transmitted light, arising from variations in thickness or refractive index, into visible differences in brightness. It thereby reveals transparent features that produce insufficient contrast under ordinary bright-field illumination. Closing the condenser aperture or defocusing can improve conventional contrast but reduces resolving power, while dark-field illumination chiefly emphasizes sharp boundaries and may inadequately display gradual transitions or details within inclusions. Phase contrast employs an annular illuminating diaphragm and a corresponding phase plate in the objective to modify the relationship between direct and diffracted light. Positive phase contrast renders optically denser features darker than their surroundings, whereas negative phase contrast reverses this relationship. Conventional equipment initially presented difficulties because its high-power objectives had insufficient working distance and depth of focus for inclusions situated within relatively thick gemstones. With assistance from H. Piller of Zeiss-Winkel, Gübelin obtained a specially designed arrangement incorporating suitable illumination components, annular diaphragms, and objectives of lower magnification with longer working distances. An auxiliary microscope permits alignment of the illuminating ring and the objective’s phase ring. The arrangement allows convenient changes between phase contrast and conventional illumination, facilitating comparison and interpretation of the same features. Its principal value lies in increasing the visibility of inclusions and compositional irregularities, rather than providing quantitative refractive-index measurements. Comparative examples include muscovite flakes in yellow beryl from Madagascar, structures in a Colombian emerald attributed to incomplete calcite exsolution, alteration to pinite in Sri Lankan iolite, and partly healed fissures in sapphire, fluorite, and beryl. Phase contrast makes the boundaries and internal details of muscovite conspicuous despite its small refractive-index difference from beryl. It also distinguishes residual liquid-filled portions of healed fractures from the surrounding healed material and can reveal gas bubbles within those residual cavities. Structural irregularities and zoning may become visible as contrasting light and dark areas where exsolution, precipitation, alteration, or healing has produced slight local differences in refractive index. Gübelin emphasizes that the method is most useful when optical differences are small and the relevant structures sufficiently thin. It offers little advantage for many ordinary inclusions whose refractive indices differ greatly from those of their hosts, and it does not reveal strain or deformation caused by stress. The article presents phase contrast as a valuable addition to gemological microscopy for investigating internal heterogeneity and interpreting processes recorded within gemstones.