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Collectors lusting after the rarest of gems have a new kind to add to their list: diamonds that change colour when made extremely cold.

These aren鈥檛 the first gems known to change colour: so-called 鈥渃hameleon diamonds鈥�, first discovered in 1866, switch their hue from greenish to either brown or yellow when heated to about 200掳C or kept in the dark for more than 24 hours.

But while analysing diamonds for clients, Stephanie Persaud at the Gemological Institute of America (GIA) in Carlsbad, California, discovered a new kind of colour-changer that goes from grey to yellow when chilled to the temperature of liquid nitrogen, or -196掳C. The GIA calls these 鈥渃ryogenic鈥� diamonds.

This cooling is done routinely in the lab to make the atoms in a diamond vibrate less, allowing more precise measurements of how the gem absorbs light of different wavelengths.

Even though few owners of diamonds will store them in liquid nitrogen, these stones might still be very valuable, says Paul Johnson at the GIA. 鈥淐onsumers love rarity, that is what brings value to a diamond.鈥�

The discovery might help explain how chameleon diamonds change their appearance, which is still something of a mystery. In general, diamonds show colour because of a disturbance of their crystal lattice of carbon atoms. This is usually caused by the presence of atoms that don鈥檛 quite fit, such as nitrogen or boron.

Johnson suspects that when cryogenic diamonds become cold, electric charge is moved closer towards or further away from such impurities in the crystal, meaning it plays differently with impinging light. To find out if this is the case, Johnson hopes to discover more than the five examples his team has unearthed so far, some of which go from grey to yellow and others from grey to blue.

Until now, no one has been watching out for colour changes in chilled diamonds, so it is possible that a number of them will be found through the GIA鈥檚 records of the many diamonds it has graded in the past.