Grandidierite is an extremely rare greenish blue orthorhombic borosilicate with the formula (Mg,Fe)Al3(BO3)(SiO4)O2. It is the magnesium end member in a solid-solution series where the stoichiometric concentrations of Mg and Fe will vary and determine the ultimate nomenclature of the mineral. Ominelite is the iron end member. The structure of grandidierite is made up of BO3 triangles; SiO4 tetrahedra; distorted AlO5, MgO5, and FeO5 trigonal bipyramids; and AlO6 octahedra.
Grandidierite characteristically forms in translucent bluish green or greenish blue elongated crystals but is rarely very clean. It can be light to dark green, or light blue or dark blue-green, or green-blue. Its coloration may resemble blue Apatite, Kornerupine, or Paraiba Tourmaline and the blue components increase with the Fe content. Crystals of up to 8 cm in length have been reported but facet-grade rough is extremely rare. Grandidierite possesses good cleavage along the {100} plane and is strongly pleochroic. It is inert to both long and short wave fluouresence. Crystallographic orientation and critical angles will impact the color of most faceted gemstones. In Grandidierite, where the pleochroism is strong, the effect may be dramatic and depending on the orientation, the stone may face up green or blue or a mixture of the two.
Although Grandidierite has been reported from localities in New Zealand, Norway, Surinam, Italy, Malawi, India, United States, Canada, Antarctica, the Czech Republic, and Sri Lanka; South Madagascar is the only source producing gem quality stones.
Grandidierite is considered to be one of the rarest minerals on earth and fine stones are always in demand. It’s a gem collector’s dream … not only for its extreme rarity but also for its relatively high hardness and the strongly saturated neon colors in which it may occur. With a hardness of 7.5, Grandidierite is hard enough to be set up as a center stone in any kind of jewelry. And, because of its intensive coloration, its one of the few collector stones that’s actually beautiful as well. Yes, beauty, rarity, durability, and portability, are attributes that so aptly describe gem Grandidierites and put them near the top of every wish list.
According to Forbes (Nov 2, 2015), Grandidierite was rated as the 3rd most expensive gemstone in the world and was valued at prices up to $20,000/ct. Although cab and mineral quality stones are not uncommon, clean facet quality stones are still almost unknown. Before 2014, a single .29 carat stone from Sri Lanka was apparently the only reported facet quality stone. In the world of gemstones, bigger is better and since large clean Grandidierites are nearly nonexistent, they are highly prized and commensurately valued - especially if they are clean.
Grandidierite was initially discovered on the southern coast of Madagascar in an area known as Cape Andrahomana in a pegmatite near Tolianaro (formerly Fort-Dauphin). Very few if any gem quality stones were ever produced at this initial deposit and in any case, they’re all gone now. Grandidierite has also been found 30 km to the north, near Vohibola, and at the Nampoana quarry near Tolanaro, and at Sakatelo, Ampamatoa, Marotrana, Sahakondra, and Ihosy. But most of these additional localities are of geological interest only as they are only said to have produced microscopic crystals.
A new an important deposit of Grandidierite was discovered in 2014, 80 km northeast of the initial discovery at Cape Andrahomana and some 15 km from the village of Tranomaro. The area is located in the Amboasary district of southern Madagascar’s Anosy region. Access from Tolanaro is via a 60 km paved road west to Amboasary Atsimo followed by a potholed unpaved 50 km stretch to Tranomaro. From there, the deposit can only be reached on foot after a half day trek.
Mining is done by diggers using artisanal and small-scale mining methods along with spades, pickaxes, and shovels. The rough crystals are extracted and sorted on-site. Due to the locality’s remote location and its discontinuous veins, production is expected to be limited and could evaporate at anytime. The deposit itself extends over a few acres and the weathered pegmatite is exploited near the surface where it is easiest to work. Mining continues as of this writing.
In late 2016, the first ever larger facet quality stones began to appear in Bangkok. A near clean stone weighing over 5 carats was observed along with a few larger but less clean stones including one over 10cts. As Grandidierite is one of the rarest gems in the world all of the stones were snapped up in a flash.
At Cape Andrahomana, where grandidierite was first described, it was associated with quartz, microcline, red almandine, pleonaste, andalusite, and biotite in a pegmatite. It as also been found as a rare accessory mineral in aluminous boron-rich pegmatites; in aplite, gneiss, and crystalline rock associated with charnockite; and in rocks subjected to high-temperature low-pressure metamorphism. Grandidierite has been associated with white plagioclase and black Mn oxides, and with phlogopite, enstatite, diopside, chlor-fluorapatite, sapphirine, dravite, aplite, serendibite, sinhalite and diopsidite as accessory minerals . Inclusions of zircons and monazites along with two-phase liquid-vapor fluid inclusions have been observed.
The crystalline bedrock of Madagascar is classified in terms of geodynamic sections that define constituent elements of the Precambrian Shield. These are, from southwest to northeast, the Vohibory, Androyan-Anosyan, Ikalamavony, Antananarivo, Antongil-Masora, and the Bemarivo domains. Grandidierite-bearing pegmatites are found only in the Anosyan domain.
The Tranomaro deposit has produced more than 800 kg of rough specimens, but only some 60 grams of eye-clean crystals. This is why gem quality stones are so rare. The ratio of gem-quality crystals to rough is about 1 in 10,000. With that kind of rarity it is no surprise that Grandidierite prices can equal or even exceed prices of bixbites, benitoites, and alexandrites.
