Lumps of gold molded into rings, coins and ingots have been highly prized for millennia. But recently, scientists have realized that tiny pieces of this precious metal – far too small to be seen by the naked eye – could also become a valued commodity. In labs around the world, gold nanoparticles are being tested as components in technology and medicines. See how gold could be used to kill cancer cells, improve the efficiency of solar cells and catalyze chemical reactions.