This snake with two heads is 43 cm long from nose to nose, and was worn on the chest, probably by an important person during a religious ceremony.
It was made between 1400 and 1500 by the people we call Aztecs, but who called themselves Mexica (say 'Meh-shee-ka').
The snake's make up
The double-headed snake looks as if it's made of stone, but is actually made of wood covered with mosaic - small pieces of coloured stone.
This mosaic is made from turquoise (a blueish mineral), stuck on the wooden base with tree-resin gum.
Oyster shell was used for the red around the snakes' heads, and conch shell for their white teeth.
The eye sockets still have traces of beeswax, showing where the snakes' eyes, possibly made from gold-coloured iron pyrites, were stuck in.
The snakes' heads have mosaic both sides. The back of the coiling body is undecorated, though originally it was probably painted gold.
People are often scared of snakes, even though most of these reptiles are harmless.
To many ancient peoples, the snake was a symbol of magical power, mysterious and deadly. To the Aztecs, snakes were important in art and religion.
One of their gods was Quetzalcoatl, the 'Feathered Serpent' and they had other serpent gods too.
Snakes shed their skins each year, perhaps suggesting rebirth and change to the Aztecs. Also a snake slithers with ease between ground, water and forest trees - just as a god might move between earth, water and sky.
The Aztecs founded their capital city at Tenochtitlan (Mexico) in 1345.
They conquered their neighbours to create an empire, and by 1500 they had made Tenochtitlan one of the largest cities in the world.
Conquered peoples paid taxes to the Aztec emperor. They sent gold, feather-work and mosaics like this serpent ornament as 'tribute'.
Such riches were a tempting prize for the Spanish soldiers led by Hernan Cortés, who landed in Mexico in 1519. The Spaniards captured the Aztec emperor Moctezuma, who was later killed, and by 1521, they had destroyed the Aztec empire, and seized much of its fabulous wealth.
Only a few Aztec treasures, like the double-headed serpent, survive.