Despite its name, the Black Prince’s Ruby is not a ruby, which is why it is known as “the great imposter”. The gemstone is actually a blood-red uncut spinel, which was named after the “Black Prince”, Edward of Woodstock, Prince of Wales.
At that time, all red transparent gemstones were thought to be rubies, and it was only through technological advancement of mineral studies that the Black Prince’s Ruby was discovered to be red spinel.
While ruby and spinel share some physical characteristics, ruby can be distinguished based on its hardness and density as it is slightly harder and denser than spinel. Fine red spinels are considered to be rarer than rubies and can often be even more valuable.