A cong (or tsung, which is how it's pronounced) is a mysterious large jade object found in very ancient Chinese burials. They are associated with the Liangzhu culture of about 2500 BCE.
It's described as a cylinder but it looks like a tall square vase: about half a metre high. In fact it's a cylinder on the inside, with a series of square layers all the way down it, separated by narrow gaps.
On each of the four faces of each square projection are shallow horizontal grooves, and some dots. This makes it just possibly look like a physical instantiation of an I Ching - except that the patterns are too symmetrical: they don't vary to give different hexagrams.
The objects have no conceivable function, and are quite unlike any other decorative object known. They wouldn't work as musical resonators, for example. Yet pieces of solid jade this large in Neolithic times must have been of enormous value, so they must have been for very high status.
The markings are thought to be highly stylized faces. Earlier congs may have more recognizable human and animal marking. So they may be a kind of miniature totem pole, a representation of the cosmos. Later ones are much more abstract and unrecognizable, and by the time of the Han Dynasty (200 BCE) they were already regarded as mysterious and ancient collector's items.
Having just seen a couple in a museum, I first thought they must be from the highly advanced periods of classical Chinese civilization: an elegant Song dynasty vase perhaps. But, to quote one Angus Forsyth in a book Jades of China, "There is a fearsome, cold reality ..
about the high precision detail and volumetric organization of the two register congs which sets them far apart from, and above, all other neolithic jades." - The website quoting this then remarks that the word 'neolithic' can be dropped. They are spooky objects by any measure, and doubly spooky when you think they're neolithic.
See also
http://www.chinese-gifts.com.hk/jade/hist.htm
http://users.exis.net/~jnc/nontech/jadetech.html