It was a gem of a find. A diamond that was recently extracted from a mine in Yakutia, Russia, had a surprise lurking inside: a tiny, second diamond.

The outer diamond measured about 0.2 inches (4.8 millimeters) long, while the wee stowaway spanned about 0.08 inches (2 mm) long and weighed about 0.0001 ounces (0.004 grams). The hidden gem rattled around inside an air pocket at the heart of the larger stone, and Russian experts who examined the peculiar double gem declared it the only known example of a diamond with another diamond inside it, according to a statement.

This unusual diamond-in-a-diamond drew comparisons to a traditional Russian toy called a matryoshka doll, in which successively smaller wooden dolls are nested inside bigger dolls, representatives with the Russian diamond mining company Alrosa told Live Science in an email.

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