Spinel, the stone that was mistaken for ruby
Some of the most famous "rubies" in royal collections are red spinels. Mineralogists only began telling the two apart in the late 1700s.
Key points
- Red spinel and ruby look alike and turn up together in the same gem gravels.
- Under a dichroscope, ruby shows two slightly different reds and spinel shows only one.
- Spinel suits a daily ring, and fine red spinel costs less than a similar ruby.
In this article
The large red stone at the front of the British Imperial State Crown is called the Black Prince's Ruby. It is a spinel. According to tradition, it was given to Edward of Woodstock, the Black Prince, in 1367, and it has been called a ruby for most of the centuries since.
The mistake was easy to make. Red spinel and ruby look alike, and they turn up together in the same gem gravels in Myanmar and Sri Lanka. For centuries red stones were sorted by color, and the red spinels from the mines of Badakhshan, in what is now Tajikistan and Afghanistan, were sold as "balas rubies".
Telling them apart
In 1783 the French mineralogist Jean-Baptiste Romé de l'Isle described spinel as a mineral separate from ruby. Ruby is corundum, an aluminum oxide. Spinel is a magnesium aluminum oxide with a different crystal structure.
A gemologist can separate them in minutes. Light passing through spinel travels as a single ray, while ruby splits it into two. Because of that, a ruby shows two slightly different reds when turned under a small instrument called a dichroscope, and a spinel shows only one.
Spinel on its own merits
Spinel is 8 on the Mohs scale and tough, so it suits a ring worn every day. It comes in red, hot pink, orange, purple, gray and blue, including a rare cobalt blue. Most spinel is sold untreated, which is unusual among colored stones.
Fine red spinel costs less than ruby of a similar size and color, and it has its own collectors now rather than being a substitute. In 2016 it was added to the American birthstone list for August.
Filed under Stones · July 2, 2026
