Tektites are small beads of silica glass that are usually black but can also be shades of brown or green. People used to think they came from the Moon, but now we know they are from the Earth. Tektites formed during impacts on Earth when plumes of vapour and melted rock pierced the atmosphere. The impact debris travelled briefly through space before reentering the atmosphere--spreading the tektites over huge areas called strewnfields. The largest tektites, which are found on land, are several centimetres across. The smallest, which are found in layers of deep sea sediments, are only micrometres across.