(IDEX Online) - Luxury French jeweler Boucheron is using Aerogel - the lightest substance on Earth - in place of a center stone in its Goutte du Ciel (a drop of sky) necklace.The substance is 98.8 per cent air (and 0.2 per cent silica) and has been used by NASA top insulate its Mars Rover Unit. It is a synthetic material, in which gas replaces the liquid, and ounce for ounce it is more expensive than gold.Claire Choisne, Boucheron's artistic director says the necklace breaks the codes of high jewelry and "highlights the incredible beauty of simple moments such as the passing of a cloud, the caress of a feather or the swirling of birds."She has also used recycled industrial waste including a processed and harmless derivative of asbestos - usually crushed and used for road-building - in her pieces.The Goutte du Ciel necklace features a 6cm Aerogel pendant encased in rock crystal and is set with 6,162 diamonds (108.17 tcw), with a $674,000 price tag.