Depends which of many rumours you are referring to, but it is certain that there is a skull known as the Mitchell-Hedges skull which is composed of a near perfect piece of clear quartz with a level of craftmanship that is believed to be unobtainable using present technology. It is also believed that the skull is aincient in origin, and it's owners claim to have found in South America (British museum dispute this). Who and how it was made remains unknown.
http://www.dreamscape.com/morgana/nereid2.htm
One of the 13 aincient skulls can be seen in the British Museum, although they have suggested it is a 'fake' after finding tool marks on a part of the artefact.