The main difference between glass and crystal is the lead content. Glass is composed of sodium and potassium. Crystal is also composed of sodium, but in addition, it contains silicone, and has a lead content of 10-24%. The lead crystal makes the glass heavier and, most importantly, the lead changes the refraction index of the item, which makes it appear brighter and cleaner. Because the refraction index is higher, and lead crystal is not as brittle as standard glass, it is possible to make deeper and more complicated cuts in crystal, creating sparkling collectibles, like those produced by Swarovski Crystal.
The presence of lead in crystal softens the glass and makes it more accessible for cutting and engraving. Lead crystal is made from a mixture of sand, potash (potassium carbonate) and lead oxide. The components are heated to at least 2,400 degrees Celsius until molten and red-orange in color. The temperature is then slowly reduced to a working temperature. Glass can contain up to 40% lead, if maximum hardness is desired. On the other hand, crystal can contain less than 24% lead if it has a high proportion of barium oxide, which ensures high quality light diffraction. Most Swarovski Crystal has a 32% lead content.