In a Nutshell
Lead. Crystal—unrelated to naturally occurring crystalline solids—is really glass with 24% or higher lead added to it. For that reason it is also known as lead glass. Sometimes glassware is made with less than 24% lead in which case it is called crystal glass. Only experts can really tell the difference at a glance, but real crystal-ware is heavier than glass and rings brightly when tapped.
The Whole Bushel
Lead crystal is not the best term to use when referring to lead glass because true crystal has a crystalline structure whilst lead glass (and regular glass) are amorphous solids. The health risks of lead mean that much of todays lead glass is actually manufactured with substitutes for lead which makes true lead glass more valuable due to scarcity. Some companies (such as Swarovski and Waterford) still use real lead which is why their crystal is so expensive.
Historically lead glass decanters were used by the rich to store wines such as port and sherry. The long exposure to the lead in the glass may be the reason for the rich man’s disease gout and there are even studies that have linked crystal glass use to human deaths. Generally glassware made from lead is safe if used for relatively short periods of time—so you are unlikely to get sick drinking out of a Waterford Crystal glass.
Lead oxide (the vital ingredient in lead glass) has been used in glass manufacturing since at least 1400 B.C., with the oldest known example being a fragment of blue glass from Nippur, an important Ancient Sumerian city. In the Middle Ages lead glass was frequently used in stained-glass windows to mimic the appearance of jewels.