Stone under the microscope – The Pearl Granites

Of the many natural products pearls have traditionally attracted considerable attention for their rarity and aesthetic appearance. Pearls without blemish have symbolized perfection since ancient time. They were so highly esteemed in classical Rome that permission to wear them was restricted to persons of high rank. The Roman statesman Pliny considered pearls as “the richest merchandise of all and the most sovereign commodity in the world”.

The key factors to the desirability of the pearl are iridescence and translucence. Added to this is its rarity and fragility requiring delicate workmanship and maintenance to retain its splendour and value.

Because of this traditionally high regard for the intrinsic characteristics of pearls it is little wonder that the occurrence of another natural product with similar characteristics commands an equally esteemed status in the field of stone. History has it that the “Pearls of Norway” remained undiscovered until an exhibition at the World Fair in the 1890’s called for each participating country to submit and display the two most prestigious varieties of building stone produced in each country which were then to be judged by a panel of architects. Embarrassed, (because Norway only quarried one variety of stone at the time – presumably marble) an urgent request was sent out to a scientist to find another type of stone for this exhibition. Having spent much of his time around the Larvik region of Norway, Broegger submitted a larvikite to the government for the event. It is recorded that a variety of Blue Pearl from Larvik won first prize. It has maintained its high stature since that day.

Clearly cashing in on the prestige afforded by the Norwegian Pearls numerous stones containing some iridescence in the alkali feldspar have been renamed and/or marketed as a “Pearl”. Black Pearl is a variety that is widely distributed. The Black Pearl that is now freely available is quarried in India and exported worldwide whereas the original Black Pearl from Sierre Leone, West Africa has virtually disappeared and could not be procured for examination. The Indian Black Pearl is a mineralogically, texturally and structurally complex rock containing abundant alkali feldspar, a scattering of plagioclase feldspar, and minor quartz. The ferromagnesian minerals are very enriched in iron and include subequal amounts of calcic pyroxene, ferrohypersthene, ferro-edenite or hastingsite, plus minor possible annite (mica).

Apatite is conspicuous and locally abundant as moderately large crystals. Modally it just falls into the syenite field, adjacent to monzonite. Textures show that this type of rock has had a long and complex history. Deformation textures are common and there is good evidence for both high-temperature and subsequent low-temperature thermal events that have modified the original mineralogy. Intercrystalline micro-fractures are very abundant as are veinlets of chlorite through the alkali feldspar. These veinlets are largely responsible for the dark green colour together with the very dark green ferromagnesian minerals. Both feldspars contain an abundance of tiny, oriented inclusions, commonly in three directions, and of different compositions in the respective feldspars. It displays little or no iridescence, labradorescence or schiller effect so should not be part of this “family”. An interesting pearl granite being exported from Finland is Lilac Pearl, a gneissic granulite containing large feldspars and a strange mineral called nuumite. Previously discovered in Greenland this mineral is commonly known as Greenland opal.

Other pearls available on the world market are prefixed with the following: silver, golden, green, pacific, aqua, brown, lunar, yellow, and oyster. For something different just swap the names to give pearl green, pearl flower, pearl red, pearl white, pearl grey, pearl star and even pearly green. When unable to categorize the stone just call it grey blue pearl. Many of these “recent” pearls emanate from Asia. The calcareous category of stones also has some pearls, such as Sinai pearl and Sicilian pearl. Even Australia has a variety known as Snowy River Pearl. This is quarried near Omeo, NE Victoria, and is a spectacular granitic rock of overall pinkish-brown colour containing large to very large phenocrysts of prismatic, randomly oriented alkali feldspar crystals that display a strong bluish iridescence in certain orientations.