What is a carbuncle gemstone?
By Jared James · Last updated 21 May 2026
Definition
In jewellery, a carbuncle is a red garnet cut en cabochon, meaning it has been shaped into a smooth, polished dome rather than being faceted. The term comes from the Latin carbunculus, meaning a small glowing coal, and it was historically used for any deep red stone cut this way, particularly red garnets and spinels. You encounter the word mainly in historical and antique jewellery contexts, especially pieces from the Georgian, Victorian and medieval periods, where cabochon-cut red stones were common.
Frequently asked questions
- Is a carbuncle a type of gemstone?
- A carbuncle is a cut and style rather than a specific mineral species. Traditionally it refers to a garnet, most often a deep red almandine or pyrope garnet, polished into a dome shape. The word has also been used loosely for other deep red stones cut the same way.
- What is a carbuncle in Sherlock Holmes?
- In Arthur Conan Doyle's story The Adventure of the Blue Carbuncle, the carbuncle is a large, brilliant-blue gemstone rather than a red garnet. Doyle used the word in its broader historical sense of a precious stone of great beauty, which adds to the mystery of the tale.
- How do I know if an antique red stone is a garnet carbuncle?
- Look for a smooth domed surface and a deep, warm red colour. Antique cabochon garnets often have a slightly irregular underside and a foil backing in older settings. A gemmologist can confirm the species and cut with certainty.
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