Black Sapphire

Even though these gems look very dark at the first sight, if light is emitted through it , they glow in dark blue color. These have refractive index range from 1.763 to 1.768 and gravity of 3.95 and 4.03. 

Reason for this is due to lower temperature of inner crust around Sri Lanka compared to other areas on earth. Key chemical substance in Black Sapphires are Aluminum Oxides (Al₂O₃). When gem is forged naturally , These molecules bind densely in them giving that dark color. You can purchase true black sapphires with the help of gem experts. Otherwise there is a very high probability of having fake blue sapphires in your hands.