I have one too. Mine is made of a kind of resin.
It is quite a good copy of the lid of the sarcofagus of king Pakal, the famous ruler of Palenque.
He is descending down into the underground, into the jaws of the underworld god/beast (forgot the name).
Above him the mayan tree of life is depicted. It looks like a cross.
Von Daniken thought he was an astronaut.
Great pic by the way. Enjoyed it very much!!!