Lead crystal is lead glass that has been hand or machine cut with facets. Lead oxide added to the molten glass gives lead crystal a much higher index of refraction than normal glass, and consequently much greater "sparkle". The presence of lead also makes the glass softer and easier to cut. Crystal can consist of up to 33% lead, at which point it has the most sparkle. The higher lead content also makes it much more difficult to form crystal during blowing.
Englishman George Ravenscroft discovered crystal in 1676.