December 23, 2018
Composition of Emerald Rough
Rough emerald is a green member of the beryl family (Be3 AI2 Si6 O18). Its fine, intense green color comes from traces of chromium and vanadium replacing the aluminum in the beryl. The most valuable emeralds are a medium lustrous green. If an emerald has 1/ 100 of 1 percent too much chromium, a very blackish-bluish green emerald forms. Similarly, a 1/100 of 1 percent too little in the chromium count could cause a rather yellowish green stone to form.
Most emeralds with intense green color have inclusions (internal flaws) due to the element replacements that occurred during their formation. These inclusions are usually acceptable, and even add value to the emerald, IF they do not diminish the stone’s durability or beauty.
Color: Emerald green, light green, yellow-green, dark green, bluish tones. Overall, an emerald’s color is very stable against heat and only changes at 700-800 degrees Celsius.
Refractive Index: (the angle which light is bent travelling through the emerald):
Low 1.577 (+/-.016) , high =1.583 (+/- .017); average 1.57 – 1.60
Mohs / Hardness: 7.5 – 8 – emerald is a hard stone, but needs care due to the internal structure’s liquid filled inclusions and fractures. The chromium and vanadium in the stone weakens the crystal lattice and produces a highly flawed structure, making it prone to damage.
Density (specific gravity): 2.72 (-.05, +.12)
Cleavage: one direction, poor.
Crystal system: hexagonal
Pleochroism: Strong, green and blue/green
Fluorescence: Longwave = Inert to Weak; Shortwave = Inert to Weak
Environment: Beryl develops in pegmatite and certain metamorphic rocks.
Treatments: Rough emerald is often treated with cedar wood oil to mask flaws, although many emeralds mined in the Swat region of northern Pakistan are of such high quality, no treatments are needed. It is this very reason that Pakistani emerald now commands the best prices for high quality, untreated (100% natural) emerald.