I have been researching alternate methods to measure birefringent values when a stone tests “Over The Limit” (OTL) using the standard critical-angle refractometer. How do we measure birefringence with a R.I > 1.81?
Two questions:
Is it necessary to use the numerical birefringence values/range for stone ID with an R.I. above 1.81, or is the optical behavior and sign sufficient? (I.e. Uniaxial, Biaxial, +/-; observed with a polariscope.)
What instruments or methods can be used to measure the birefringence?
I have found several that can be used/purchased or modifying a microscope by adding a vernier/micrometer apparatus to measure the numerical values. The article linked above describes the use of a vernier/micrometer to analyze/measure the R.I. of stones above 1.81, mainly for singularly or uniaxially refractive specimens, but what about stones that are double refractive?
Would like to understand if/how the numerical values/ranges for Double Refractive stones could be used for those that are OTL and if pursuing instruments that can be used/fabricated is a good approach or not.
If you were to run a laser beam through the stone and aimed the resulting beam a particular distance from a blank wall, you are able to see the doubling effect projected onto the wall.
With the right formula you could extrapolate the value of birefringence as it splits the laser beam into two.
Using a laser would work if the gemstone is flawless and transparent to the laser wavelength. Diamonds would be a good candidate if they displayed birefringence.
Any stone that is semi-transparent to the laser and/or with moderate to heavy inclusions might induce dispersion (scattering) of the beam diameter. Calculating the center of the split beams with unknown dispersion will require some curve-fitting math to find the center of the beam spots on the wall in order to measure the distance between.
The abstract has some information on an analysis method referred to as “MultiPol” being used in the diamond industry. The author talks about using HPHT diamond material for experimentation. I suspect the entire thesis is not available due to proprietary information, since the author’s thesis was partially funded by the Diamond Trading Company (DTC).
I found another research paper from same author talking about similar research… here
Still digging through the mass of papers in this rabbit hole…
The diamond and birefringence subjects seem to be juxtaposed a lot. Gemology seems to keep diamond as non-birefringent yet the metrology world seems to think otherwise.
Hi Troy! I really enjoy your posts… re diamonds:this is my three cents: theoretically, isometric (cubic) crystals structures are singly refractive. They should not display bifrefringence… internal strain during crystal growth distorts the crystal structure in a similar manner that bending a stick of plastic does…the amount of internal stress along cystal dislocation faces at the microscopic level can be quantified by the degree of splitting of polarized light. Petrological microscopes have long used crossed nicols prisms for mineral ID in thin sections of rock… now replaced by Glanz Thompson prisms with polarization at 90 degrees and an air gap between the two prisms which amplifies light beam splitting…The limitations are that optical calcite has a lower refractive index than most gemstones…making hand held refractometers unuseful for measuring the RI of highly refractive stones…petrological microscopes use both transmitted and reflected light overcoming some of the obstacles with calcite. Triclinic crystals have three nonorthogonal axes of different lengths complicating measurements… These include microcline and plagioclase feldspars as the most common minerals (amazonite, labradorite gems) that are highly birefringent… rotating the prisms and measuring the angle of extinction with transmitted and reflected light helps ID minerals… partial exinction with high RI minerals can still be measured. I am more familiar with the use of petrographic microscopy to identifiy poly mineralic rocks rather than single mineral gem polarized light microscopy… the modal analysis and classfication of igenous rocks is time consuming and tedious… thin section microscopy counts individual mineral grains, ID’s them with optical properties, and estimates the volume percent of each type of mineral from the area covered in thin section of each mineral… a task that is relegated to upper division undergrad and grad students… The mode gives the rock type by it’s actual mineral content… Normative analysis gives the rock type by chemical analysis with all components expressed as metal and silicon oxides (silica) and the mineral content reconstructed using an algorithm to make minerals out of the oxides… another tedious process.
Just for interest: The use of polariscopy has been extended to biological materials. proteins show birefringence…and to clinical medicine-- which you might find surprising… My former eye doctor used polarization to do optical tomography and make a picture of my optic nerve head with the nerve fiber layers…
ophthalmology, binocular retinal birefringence screening of the Henle fibers (photoreceptor axons that go radially outward from the fovea) provides a reliable detection of strabismus and possibly also of anisometropic amblyopia.[22] In healthy subjects, the maximum retardation induced by the Henle fiber layer is approximately 22 degrees at 840 nm.[23] Furthermore, scanning laser polarimetry uses the birefringence of the optic nerve fiber layer to indirectly quantify its thickness, which is of use in the assessment and monitoring of glaucoma. Polarization-sensitive optical coherence tomography measurements obtained from healthy human subjects have demonstrated a change in birefringence of the retinal nerve fiber layer as a function of location around the optic nerve head.[24] The same technology was recently applied in the living human retina to quantify the polarization properties of vessel walls near the optic nerve…
Thanks! I enjoy and learn a lot from your posts as well!
There seems to be many things that are not apparent in open sources on how birefringence is measured using instruments such as a Petrologic Microscope or other interferometric devices.
Obviously, any material with an R.I. between 1. 35 - 1.78 (am providing some margin boundaries for low/high values) can be measured accurately with the standard Gemological Refractometer.
As you mentioned, Petrological Microscopes have good methods observing polarized light transmittance through samples lapped to specific thicknesses. But what is not apparent is how those observations are translated into numerical values. Do you have any references on the subject? (textbook titles, white papers, etc.)
I recently came across this link: Principles of Optical Birefringence which is a fantastic reference on the subject. But like others I have found, the content is written as a top level view, peppered with some details for flavor, but the whole recipe is… missing.
Are we looking at another Coke-Cola proprietary secret ingredient dilemma here or am I just missing keyword search phrases?
unfortunately petrological microscopy is not suited for whole gem optical analysis… thin sections sliced, mounted on glass slides and polished down to 30 microns thickness for transmission and reflected dark field microscopy is used to identify minerals according to refractive index… major axes of crystalline grains determined by angle of extinction with crossed polarizing filters…a rotating stage can zero in on a major axis…almost all minerals except cubic have birefingence due to anisotropy from three axes set at different angles…numberical values for common rock forming minerals are available in tables and data sets for these mineral by major axis angle of extinction. Need to reference petrological data sets. What I find that is actually more interesting but not in this field is the application of polarized optics to living cells and biological systems… proteins and other biomolecules are all birefringent…
That aligns with the lack of information I have found so far on using the petrologic instruments for measuring birefringence.
Still haven’t found any articles or papers that address measuring the birefringence of gemstones above the OTL RI, yet. This is a very interesting puzzle. Somewhere… someone or some entity has the golden ticket to this conundrum. The AI queries I have thrown out into the ether have resulted in a plethora of articles / papers on the biological and material testing applications of birefringence, but the gemology world seems shrouded in nebulosity.
Although this belongs on another thread where I advocated for a formal licensure for gemologists, there is a actual post doctoral degree in gemology granted by the University of Nantes in wesern France.
Emmanuel Fritz is a current professor of physics at the University of Nantes in western France. He recieved his PhD at the Sorbonne in Paris, worked for 10 years with the GIA in the US, then return to take a professorship at Nantes… he was instrumental in founding the “diplome d’Universite de Gemmologie” at Nantes where the program celebrated it’s 20th date of founding. His mission, as is part of the GIA’s mission (the rest stated as bring order to the gem market), was to put gemmology on a scientific basis… This degree is a post doctoral degree/fellowhip equivalent in the US and is recognized throughout Europe as being an academic degree.
The GIA is the closest analogue to this degree that is avaiable in the US, but it’s not an academic degree, despite being the most respected and rigorous. It really behooves the GIA to partner with universities in the US to offer a 6 year PhD degree in gemmology as an academic degree as an option to their current certificates. The current program can continue, but added academic credentials would give even further credibility to gemmologists. That would still allow non college graduates a certificate but allow those interested in advanced stuy to participate with an academic degree. Most of the research work at the GIA is being conducted by post docs…Nonetheless, I still feel that licensure is the way to go.
numerous European and North American geochemical and mineralogical societies sponsor the news letter ELEMENTS, of which I am a subscriber thru membership in the European Geochemical Society… The current issue is about mineral florescence…one article is about diamond florescence… I haven’t parsed thru it yet as it just came… the material is technical but interesting…a lot of it has to do with quantum physics and energy level absorption/emission… very dense reading. The general editor is Bilchert-Toft, a name that I recognize for her work In Hawaiian lava rock geochemistry from the 1990’s…all of the papers are published by known and respected authors in their fields… every other issue there is usually one or two articles specific to gems. The last one had none on gems… wholey devoted to the development of early earth 4.5-4 billion years ago with the formation of the first primitive crust and subsequent development of plate tectonics.
This is off topic but just FYI… there is one academic degree granting program in gemmology.
the information is out there in Google scholarly articles…however, I can’t find the old fashioned hand counting of mineral grains by visual analysis and birefringence… those data sets have been replaced by machine learning and automated 3D recognition of mineral grains specific to each microcope system… the reference below is an open source data base… hand counting is now a didactic lab exercise in petrological microscopy…
PWD: A Petrological Workspace and Database tool - 2019
by D Schonwalder‐Angel · 2019 · Cited by 1 — The Petrology Workspace and Database (PWD) offers an online interactive data repository for the management and visualization of multilevel data …
Thanks, Steve! This is awesome! Thank you! Bookmarked for reading material. See, I knew I wasn’t using appropriate keyword/phrases.
The world of data collection / mining is changing at a frightening rate. AI/ML is providing a level of analysis that would have taken several people months to aggregate into a usable form. Now its a few days to near real-time processing.
This one is the the AGU… there are numerous other data bases. The use of machine learning has been around for a while with SPECIFIC scientific applications but has become generalized with AI…something that we all know already but being mentioned because it does present both
an opportunity and a threat. The threat is also obvious… there are no ethical guardrails for data mining and selling of personal data… as long as money can be made legally off of data selling, there will be no protection for privacy and personal data. Illegal use of data forces all of us to pay for extra security… what the future will hold is up for grabs as AI advances exponentially.
Thin section visual analysis took hours, counting more than one slide to be representative of a whole rock sample… now it can be done is seconds. However the utility of visual representation is still intact… many papers and presentations have pictures of slides alongside with chemical analysis… the slides are kaleidoscopic in color with polarized light… simple rocks that only have three minerals, like common basalts- plagioclase feldspar (labradorite schiller cabochons, Oregon sunstone, New Mexico bytownite sunstone being gem representatives), fosteritic olivine (peridot), and pyroxene (chrome diopside being the gem example) have their own colors under polarized light that change directionally on a rotating stage…opaque accesssory minerals such as magnetite show up as black grains… showing photomicrographs is a common practice in publications. How useful this is for gem analysis is that it isn’t, it’s just science. it give you the MODAL analysis of a rock for classification by estimating the volume of individual minerals within a whole rock sample… chemical analysis by breaking down the rock into consituent oxides and reconstituting the rock by an algorithm into a set of standardized constiuent minerals based on chemistry, gives you the NORMATIVE analysis or NORM… combining the two give you a better understanding the how the rock was formed. In Hawaii, the main mountain building stage of the islands were made out of voluminous eruptions of thoelilitic basalt… the underlying Hawaiian hotspot mantle plume cause large scale decompression melting at upper levels in the oceanic lithosphere leading to a more “primitive” magma composition. As the islands moved off the hotspot, the volcanoes became senescent. Late stage lavas shifted in composition to Hawaiite…lower degrees of partial melting in deeper crust resulted in alkali and silica enrichment. Technically, the silica content qualifies the rock as being an andesite with 57-60% silica, but the sodium content increased also maintaining a trend towards silica undersaturation… modal analysis shows Hawaiite being mostly andesine plagioclase (50-70% albite feldspar) with smaller amounts of clinopyroxene and accessory olivine…Nomative analysis showed 5% jadeite hidden within the titaniferous augite clinopyroxene…thus refecting the under silica saturated trend… very small volume rejuvenated volcanism after 2 million years extinction of main stage thoelititc eruptions continue the unsaturated trend into basanite and nephelinites. The Na content was high enough to overwhelm available silica to make nepheline instead of plagioclase. Due to the small volume of undersaturated magma, not enough was present to make for ore or gem deposits that are often associated with undersaturated rocks… However on continental or subduction zone crust, large volume undersaturated magmas can pool to form nepheline syenites and even more undersaturated rocks… these rocks can host ores and gem minerals… the example of hackmanite presented by PaulB from San Benito county is one of them. Hawaiian rocks do have gemmy high magnesium (fosterite) olivine… but the crystals are too small to cut. Oceanites are up to 40% olivine thoelititic basalt resulting in an attractive rock full of small gemmy peridot crystals within a calcic plagioclase matrix… a good sample can be polished but with some difficulty as the hardness of peridot and and calcic plagioclase vary by 1 and the plagioclase tends to cleave…larger (1cm) titanaugite euhedral crystals can be found free in cinders from late stage alkali basalt eruptions. They make attractive mineral specimens- brownish red with great euhedral crystal faces…translucent at the edges but are not of gem quality, common opal and quartz crystals are rare but created by hydrothermal activity within caldera margins… these have been thoroughly picked over by collectors… the only other specimens are native sulfur from active volcano fumaroles and selenite gypsum roses… the roses were formed from montmorillonite clays leaching out calcium when high sea levels flooded lowland valleys on the older islands, with sulfate being provided by sea water… I have a collection of roses that are invaluable, collected when I was a kid… the site has been paved over for decades by a subdivision. Otherwise nothing of gem or specimen interest in Hawaii…the geologic system is too simple…
