Kyawthuite is a rare mineral with the chemical formula BiSbO4, known to be a natural form of bismuth antimonate. It’s notable for being the rarest mineral on Earth, with only one crystal ever found, located in the Mogok region of Myanmar.^[2] This unique crystal was discovered in 2010 and weighs just 0.3 grams.^[3]

Kyawthuite is a transparent reddish-orange mineral that is monoclinic, meaning it has a crystal structure belonging to the monoclinic crystal system. It is isostructural with clinocervantite, which means it has the same structure as clinocervantite but with different chemistry, specifically as an antimony-analogue of clinobisvanite . Its rarity and unique properties make it a subject of interest among mineralogists and gem collectors. Kyawthuite is described as a deep-orange jewel with a hardness of 5 ½ on the Mohs scale, which measures the scratch resistance of various minerals. It has a specific gravity of 8.256, indicating it is quite dense, and an adamantine luster, which refers to its brilliant shine and high refractive index ranging from 1.9 to 2.5. The crystal is small, weighing only 1.61 karats.^[4] Its color can range from deep red to reddish-brown, and it forms as tiny, isolated crystals with intricate structures.^[5]

The exact geological conditions for the formation of Kyawthuite are not fully understood. However, it is believed to form in pegmatite deposits, which are igneous rocks formed from the cooling and crystallization of highly concentrated molten rock.^[6] Pegmatites are known for containing rare minerals and crystals, which aligns with the rarity of Kyawthuite.

Kyawthuite has a monoclinic crystal structure, which means it belongs to a category of crystals that are described by three unequal axes with one of them being oblique. Specifically, it has the space group I2/c, and it is isostructural with clinocervantite. Being isostructural means that Kyawthuite shares the same crystal structure as clinocervantite but differs in chemical composition; Kyawthuite is an antimony-analogue of clinobisvanite. This structural similarity to other known minerals helps scientists understand and categorize it despite its rarity.

The refractive index of a material is a measure of how much it bends or refracts light. A higher refractive index indicates that the material is more effective at bending light. Kyawthuite’s refractive index ranges from 1.9 to 2.5, which is quite high and contributes to its adamantine luster, giving it a brilliant shine similar to that of a diamond. This property makes Kyawthuite particularly attractive as a gemstone, despite its rarity.

There is a synthetic replica of Kyawthuite available. This allows gemstone connoisseurs and researchers to appreciate and study the properties of Kyawthuite without the need to obtain the single known natural sample, which is housed at the Natural History Museum of Los Angeles County.^[7] The synthetic version replicates the chemical formula Bi3+Sb5+O4 and the deep orange color characteristic of the natural mineral.