Emerald

Emerald derived its name from Smaragdos which means 'green stone'.Its green is incomparable, and is therefore called 'emerald green'.

Color: Light green, yellow green, dark green
Hardness: 7.5 - 8 Mohs
Specific Gravity: 2.67 - 2.78
Refractive Index:  1.576 - 1.582

Only the finest qualities are transparent. Often clouded by inclusions (liquid or gas bubbles, cracks, and crystals). These are not necessarily classified as faults, but are evidences as to the genuineness of the stone as compared with synthetic and other imitations. Because most emeralds are highly included, their toughness (resistance to breakage) is classified as generally poor.

Treatments
Most emeralds are oiled in order to fill in cracks so that clarity and stability are improved. The use of oil is traditional and largely accepted by the gem trade.