This is the difference between optical crystal and crystal.

Optical Crystal is extremely hard and clear. This permits specialized polishing to produce a surface that transmits light with little or no distortion, which sparkles with diamond like brilliance when cut into facets. Its precise optic effects, developed for the scientific requirements of lens makers, also make it the finest engravable surface for artistic impression. Optical crystal pieces also have the distinction of being lead free, and are available in a variety of solid forms.

Crystal refers to particularly clear, hard glass with superior ability to transmit and reflect light. Lead crystal, so named because it contains lead, is familiar to many due to its traditional use in cut glass tableware. However, there are several types of crystal, many of which do not contain any lead.

Lead oxide when added to the molten glass gives lead crystal a higher index of refraction than normal glass, and consequently greater "sparkle". There is a strong perception within the industry that lead crystal products command premium prices, so they continue to use lead oxide to inflate prices. "But manufacturers are now under pressure to adopt alternatives to lead crystal," says Professor Nick Priest of Middlesex University in the United Kingdom. "The use of lead involves risks to workers and to the environment. Countries such as Denmark and the United States are attempting either to ban or restrict its use in consumer products."

However, lead-free optical crystal glass has a refractive index higher than 1,52 which makes it especially useful for works of art to provide a luxurious character, with high lustre and light transmittance.

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