A perfect negative crystal floating in space

MAGNESIUM aluminium oxide sure is a pretty mineral. It forms spinel, a gem coveted throughout history and whose red variant is sometimes confused with ruby. It was used in the Timur Ruby necklace made for Queen Victoria. And an egg-sized lump of the stuff, known as the Black Prince's Ruby, forms the centrepiece of the Imperial Crown. Both pieces are part of the British Crown Jewels, and there's an even bigger rock, the Samarian spinel, in the Iranian Crown Jewels.

The spinel in this photo is a tiddler by comparison, but no less beautiful and certainly more intriguing. The field of view of the image is 2.9 millimetres, which makes the floating octahedron in the middle less than half a millimetre wide. But get this: it's a "negative crystal". The octahedron is the outline of a space, and what looks at first like the sides of a solid crystal are actually the walls of a void inside a bigger lump of crystal.

"This is one of the most shockingly perfect negative crystals I have come across," says photographer Danny Sanchez, who is based in Los Angeles.