Russian diamond miner Alrosa has found a small, but unique, diamond that has another gem moving freely inside it, resembling a traditional Russian Matryoshka doll.

The stone, found at Alrosa's Nyurba site in Siberia, weighs just 0.62 carat, with an internal cavity of 6 cubic millimetres holding another crystal of just 0.02 carat, the miner said in a statement.

It's the first diamond of that nature ever to be found and may be more than 800 million years old, Alrosa said, citing scientists.

"[The diamond] was studied using several methods, including Raman and infrared spectroscopies, as well as X-ray microtomography. Based on the results of the study, the scientists made a hypothesis about how the crystal was formed. According to them, there was an internal diamond at first, and the external one was formed during the subsequent stages of growth," the company said.