Prismatine — The Boron-Rich End-Member of the Kornerupine Series
Prismatine — The Boron-Rich End-Member of the Kornerupine Series
Rare metamorphic gem mineral, distinguished from kornerupine by its boron content, and a collector's curiosity
Prismatine is the boron-rich end-member of the kornerupine mineral series, with the simplified formula (Mg,Fe)4(Al,Fe)6(Si,Al,B)5O21(OH). The species was redefined and given its current name and nomenclatural status by the International Mineralogical Association in 2002, with the recognition that boron-rich material previously called kornerupine constituted a distinct end-member with sufficient compositional difference to warrant its own species name. Prismatine is rare, occurs almost exclusively in metamorphic rocks of high grade, and produces gem-quality material only in small quantities from a handful of localities. It is a collector's stone with very limited commercial availability.
The kornerupine-prismatine series
Kornerupine was first described in 1884 from material collected at Fiskenæsset in Greenland and named in honour of the Danish geologist Andreas Kornerup. For more than a century it was treated as a single species with variable composition, including variable boron content. The 2002 IMA reclassification split the series into two end-members: kornerupine, with low boron content and a formula approximating (Mg,Fe)4(Al,Fe)6(Si,Al)5O21(OH); and prismatine, with significant boron substituting for silicon and aluminium in the tetrahedral sites and a formula approximating (Mg,Fe)4(Al,Fe)6(Si,Al,B)5O21(OH).
The boundary between the two species is set at the point where boron occupies more than half of the appropriate tetrahedral site. Material above the boundary is prismatine, material below is kornerupine. The boundary is defined by composition rather than by appearance, and most material in the series cannot be classified visually; laboratory analysis is required.
Both species share the same orthorhombic crystal structure and produce visually similar prismatic crystals, with refractive indices, hardness, and optical properties that overlap substantially. The compositional separation has practical consequences for laboratory identification but is essentially invisible to the gem buyer or the collector evaluating a specimen by eye.
Properties and identification
Prismatine has a hardness of 6.5 to 7 on the Mohs scale, a specific gravity of 3.27 to 3.45, and refractive indices in the range 1.66 to 1.70 with a birefringence of approximately 0.013 to 0.017. The species is biaxial and pleochroic, often strongly so, with green and brown to colourless dichroism in the gemmy material. Cleavage is good in two directions and the species is relatively brittle, requiring the same care as kornerupine in cutting and setting.
The presence of boron is the diagnostic feature that distinguishes prismatine from kornerupine, and it is detectable by laser ablation inductively coupled plasma mass spectrometry, by electron microprobe analysis, or by other quantitative chemical techniques. None of these is available outside a properly equipped gemmological or mineralogical laboratory. For working trade purposes, material in the kornerupine-prismatine series is often described as kornerupine without further specification, on the understanding that boron analysis would be required to distinguish the two species formally.
Sources and occurrence
Prismatine occurs almost exclusively in high-grade metamorphic rocks, particularly in granulite-facies pelitic and aluminous gneisses, and in skarn-like contacts between such rocks and intrusive bodies. The boron required to form prismatine is most often supplied by metamorphic fluids derived from the breakdown of pre-metamorphic tourmaline or other boron-bearing minerals, and the species is therefore most likely to be found in terrains where boron-bearing protoliths have been carried to high metamorphic grade.
Documented localities for prismatine include Sri Lanka, Madagascar, Tanzania, Sweden, Greenland, the Czech Republic, and a small number of other metamorphic terrains. Gem-quality material is rare from any of these sources; most prismatine is opaque and of mineralogical interest rather than gemmological. The cleanest gem material has historically come from Sri Lankan alluvial deposits and from a handful of Madagascan localities, with crystals usually small and faceted weights typically below five carats.
Colour and gem character
Gem-quality prismatine ranges from colourless through pale green to brownish green and brown. The green is generally less saturated than in chrome-bearing kornerupine, and the species lacks the strongly chromium-coloured emerald-green material occasionally found in kornerupine. The pleochroism, however, is often strong, with the c-axis direction showing a deeper green and the perpendicular directions showing brown or yellow.
The cutter orienting prismatine rough chooses the table orientation based on the desired face-up colour, the same calculation made for kornerupine and for any other strongly pleochroic gem. The resulting cut stones are typically small mixed-cut or step-cut shapes, faceted to display the cleanest face-up colour and the best clarity available from the rough.
Position in the trade
Prismatine is essentially absent from the mainstream gem trade. It does not appear in standard wholesale catalogues, does not have an established price grid, and is sold almost entirely in the collector market through specialist dealers and at mineral shows. Buyers interested in the species typically seek it out for the same reasons collectors seek other rare metamorphic gem species: the rarity itself, the technical interest of the boron-bearing chemistry, and the satisfaction of owning a stone that few other people in the trade would even recognise.
Pricing is irregular and depends heavily on the seller and the specific buyer. A clean faceted prismatine of one to three carats might trade in the low to mid hundreds of US dollars per carat from a specialist dealer, with prices rising for larger, cleaner, or more strongly coloured material. The species is not a viable retail or jewellery-design stone in any volume sense; it is a collector specialty, and the market structure reflects that.
Confusion with similar species
Kornerupine is the obvious species with which prismatine can be confused, and as discussed the two are essentially indistinguishable without quantitative chemical analysis. Beyond kornerupine, prismatine can also be confused with diopside, enstatite, and tourmaline, all of which can occur with similar prismatic habit and brown-to-green coloration. Refractive index and birefringence measurements distinguish prismatine from these candidates, and pleochroism observation under a dichroscope provides additional confirmation.
Sapphirine, another rare metamorphic gem mineral with which prismatine sometimes occurs, has different refractive indices and a different pleochroic colour scheme but can also occur in pale green to brown colours that visually overlap with prismatine. The two species are distinguished readily by refractometer measurement and by pleochroism observation.
In the trade
Prismatine sits at the far edge of the practical gem trade, in the territory of metamorphic mineral collecting and specialist gemmology. The species is unlikely to appear in any volume retailer's inventory, and the mainstream coloured-stone trade has no working knowledge of it. For laboratory gemmologists, however, the kornerupine-prismatine distinction is a routine identification exercise that demonstrates the importance of full chemical analysis in confirming species attribution where the visual properties overlap. The species is one of the textbook cases used to illustrate why spectroscopy and microprobe analysis matter in modern gem identification.