Polarising Microscope
Polarising Microscope
The petrographic instrument adapted for inclusion identification and origin work in coloured stones
The polarising microscope, also known in mineralogy as the petrographic or thin-section microscope, is an optical instrument fitted with polarising filters above and below the specimen stage. In gemmology it is used to identify the mineral inclusions inside coloured stones by observing their optical behaviour under crossed polars at high magnification — birefringence, pleochroism, extinction, and interference figure — all of which are diagnostic of specific mineral species.
What it observes
Inclusions inside a host gemstone behave optically as crystals in their own right. Under crossed polars, an anisotropic mineral inclusion shows extinction four times as the rotation stage is turned. The interference colours visible at maximum transmission depend on the mineral's birefringence and on the thickness of the inclusion in the line of sight. Pleochroism — the change in absorption colour with the orientation of the polariser — is visible with the analyser removed. Refractive index can sometimes be estimated by the Becke line method against a known immersion medium. Together these observations narrow inclusion identifications down to species level in a way no single test can.
Origin and synthetic-versus-natural work
For coloured-stone laboratories, the polarising microscope is indispensable for two related questions. The first is origin: characteristic inclusion assemblages are central to the geographic-origin opinions issued by Gübelin, SSEF, AGL, GIA, and Lotus Gemology. Mogok rubies, Kashmir sapphires, and Colombian emeralds each show suites of mineral inclusions that the laboratory matches against the published reference. The second is synthetic detection: flux-grown synthetic ruby and emerald carry residual flux and seed plates whose optical signatures, observed under crossed polars, reveal their growth method.
Configuration
A gemmological polarising microscope shares its optical layout with the petrographic instruments used in mineralogy departments. Crossed polars are standard, a Bertrand lens or accessory plate allows conoscopic observation of interference figures, and the rotation stage is graduated for extinction-angle measurement. Higher-end models include compensator slots for first-order red, mica, and quartz wedge plates, sensitive-tint accessories that give the operator more refined control over retardation work.
In the trade
The polarising microscope sits above the standard gemmological stereo microscope in capability and cost. Most cutters and dealers do not own one. Coloured-stone laboratories and serious origin specialists do, and the difference is visible in the depth of inclusion description that appears on a high-end laboratory report.