Mineral Alexandrite
The chemical composition of the mineral Alexandrite is indicated by BeAl2O4, Beryllium Aluminum Oxide. This mineral belongs to the Oxides mineral class. Alexandrite is a rare color-change variety of the mineral called Chrysoberyl. It is considered as one of the rare gems in the world. The scarcity is due to the mineral’s chemical composition. Mineral Alexandrite is basically a Chrysoberyl with a splendid capability to display distinct change of colors in different light source. They are commonly used as gemstones in jewelry. But beautiful top quality Alexandrite is very rare. Occasionally from England, the smaller Alexandrite crystals are used in Victorian jewelry. Alexandrite gemstones are also considered as birthstone for the month of June.
Alexandrite emerged millions of years ago in metamorphic environment. It was first discovered in 1833 at the emerald mining regions near the Tokovaya River in the Ural Mountains of Soviet Union. It was named Alexandrite by the mineralogist Nordenskjold in honor to the Russian csar Alexander II (1818-1881) whose attainment to majority was celebrated at the said time of discovery. It became the national stone of the Csarist Russia because it exhibits both green and red hues, the principal colors of the Old Emperial Russia. The Alexandrite gems became available in the market since the time the mines of the Urals in Russia became a primary source of the said mineral. When the Russian deposits of mineral Alexandrite became exhausted, the interest for the majestic color miracle decreased, more especially, when the Alexandrite mineral mined from other sources do not display the same splendid distinct color change even when viewed transmitted light of polarizing microscope. However, after the discovery of the so-called Brazilian Alexandrite in the year 1987 in a place called Hematita in Mina Gerais, Brazil, the situation changed. This Brazilian Alexandrite exhibits distinctive change in color that is very discernable to the unaided eye and most especially in when the specimen is viewed in reflected light of polarized light microscopy.
Mineral Alexandrite possesses a splendid image when viewed under petrographic polarizing microscope. They may exhibit soft shade of red, raspberry red, or purplish-red in incandescent light or even in light as simple as candlelight. In daylight or upon using a fluorescent light, they show green or bluish-green color. Crystals when examined may appear in a variation of transparent to translucent. Alexandrite stone will show a white streak when rubbed on a white plate. Cleavage, which is clearly visible with some minor adjustments on the aperture diaphragm of polarizing light microscope, is found good in one direction and imperfect in the other direction in transmitted light. It has conchoidal fracture thus, the stone will have smooth, shell-like surface when it is broken. Hardness is 8.5 in Mohs scale and specific gravity is approximately 3.5 g/cm³. Luster may vary from vitreous to greasy in reflected light of polarized light microscopy.
When viewed under a petrographic polarizing microscope, its crystal formation commonly is of orthorhombic crystal system. Index of refraction ranges from 1.745 to 1.759. It shows weak birefringence with strong pleochroism exhibited in crossed nicols of geological polarizing microscope. It shows strong absorption band in the yellow. It often exhibits elongated prismatic crystal and tabular inclusions with parallel striations, which are found in transmitted light of polarized light microscopy. Crystal fragments and loose or rounded grains can be also found. Alexandrite minerals are commonly twinned at some angles leading to a kite-shaped or triangular form as commonly found results in transmitted light of polarizing microscopes. Repeated cyclic twinning of this mineral often produces pseudo-hexagonal forms.
Mineral Alexandrite is known to be very rare variety of chrysoberyl. They commonly occur in mica shcists and also in granitic pegmatites. Its splendid color change character is brought about by the small-scale replacement of alumina by the chromic oxide to form the Alexandrite variety. They commonly occur in gneiss, granite, and stream deposits.
