Luumaki gem beryl

The whole locality has produced about fifteen kilograms of gem beryl.

That figure comes from Seppo Lahti and Kari Kinnunen, who introduced the deposit to gemmology in Gems and Gemology in 1993 under the plain title A New Gem Beryl Locality. What is quarried from the pegmatite dike at Luumaki is strongly etched prismatic crystals of yellow, green and pale blue beryl, alongside minor facet-grade topaz and a lot of varieties of quartz.

In the laboratory the stones are unremarkable. Their gemmological properties and chemical composition are much like other pegmatitic beryls, including those from Brazil and Ukraine. What may set Luumaki material apart under a lens is its inclusions: large primary fluid inclusions surrounded by haloes of micro-fractures.

The geology is old and specific. A University of Helsinki team dates the pegmatite at 1,928 million years and places it inside the rapakivi granites of the Wiborg batholith in southeast Finland. It is a moderately evolved niobium-yttrium-fluorine system, of the topaz-beryl type within the rare-element pegmatite class, and its major mineral assemblage stays simple right through the border, wall, intermediate and core zones: potassium feldspar, plagioclase, quartz and biotite, with or without fluorite.

Only half of it is worth anything to a collector. The deposit is at least two chemically related bodies, Luumaki North and Luumaki South, and only the northern one carries the miarolitic pockets that hold gem beryl, which here means heliodor.

The chemistry records the fractionation as you go. Rubidium, caesium and thallium become progressively enriched in the potassium feldspars while strontium and barium are depleted, which is the signature of a melt concentrating its rare elements as it crystallises.