What is the X in X-mas?
a type of glass containing a high proportion of lead within crystallized quartz, used esp. for decorative items
The composition of lead crystal is 54-65% silicon dioxide (SiO2), 18-38% lead oxide (PbO), 13-15% soda (Na2O) or potash (K2O), and other oxides and it has a high refractive index.
heavy and durable glass characterized by its brilliance, clarity, and highly refractive quality. Developed by George Ravenscroft (q.v.) in 1675, it ushered in a new style in glassmaking and eventually made England the leading glass producer of the world. Ravenscroft's experimentation was supported by the Worshipful Company of Glass Sellers, a body of English retailers long dissatisfied with the quality of glass from Isola (island) di Murano, Venice