Though considered discovered in England in the 17th century by Stephen Falango, crystal developed during the 18th century. Crystal glass mainly consists of silica, potash, and, most of all, lead. To be considered as crystal, the part must contain at least 24% of lead and must have a refractive index equal to or greater than 1.545. The higher the lead density, the purer is the material, with a higher brilliance. The crystal produced in large French factories, like Lalique, contains up to 31% of lead and holds the designation of superior crystal.