Q. What makes crystal, crystal?

David Whitehouse explains the difference between the terms glass and crystal in the "Glass Glossary" (found on the CMoG website https://www.cmog.org - Research, Teach and Learn - Glass Glossary:

Crystal
A popular term for colorless lead glass which has a high refractive index and consequently is particularly brilliant. Today, the word is often used to describe any fine glass tableware.

Brilliantly clear "crystal" was originally produced to imitate “rock crystal" or colorless translucent natural quartz.

According to Harold Newman in An Illustrated Dictionary of Glass (London: Thames and Hudson, 1977), some countries establish a minimum percentage of lead in crystal. For example, in England crystal "must contain at least 24% lead oxide...and that with 30% lead is called 'full lead' or 'cristal superieur.' Recent regulations of the European Economic Community (EEC) forbid any glass to be called or labeled 'crystal' unless meeting such standards, and a special label is provided for each class." The British Glass website provides a current definition of lead crystal: https://www.britglass.org.uk/AboutGlass/TypesofGlass.html#2

There are no standards in the United States. "Crystal" tends to be a popular or marketing term and may be used to describe any fine glass tableware. It may or may not contain lead.