Where exactly is the lead in lead crystal, and why is it not visible?
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Metals act like metals because they have a very loose electron. Put a bunch of metal atoms next to each other, and the loose electrons fall off and just form a shared “sea” of electrons. That sea of electrons makes metals shiny, and it also makes them great electrical conductors and heat conductors.
When metals react with other substances, this usually changes. Oxygen is short on electrons and has a tendency to steal them, so has chlorine. So metal chemicals like sodium chloride (salt) don’t act like metals. The chlorine steals the loose electrons and the sodium and chloride now get stuck to each other to form a crystal. There is no electron sea so all the shininess and electrical properties are gone.
Glass is a mixture of various oxides of metals (silicon is the main ingredient. I’m calling it a metal here, although strictly its a semi-metal – it’s loose electrons are not quite loose enough to make it a proper metal, but the are close).
You can add various metal oxides to change the properties of glass. Pure silicon oxide melts at extremely high temperatures and is very difficult to work with. Adding fillers like sodium oxide makes it melt much more easily, so it’s much easier to make stuff with it. Adding heavy fillers like lead oxide or silver oxide makes the glass much heavier.
Finally it’s just worth pointing out that glass is not a crystal – in fact, glass is the opposite of a crystal. Crystals have all their atoms nicely aligned in precise shapes. Glasses have their atoms all over the place in a chaotic arrangement.
Lead glass is often called crystal, because its weight causes more bending of light rays. Heavy glasses are widely used in camera lenses for this reason. If you cut glass with lots of edges and facets then it will tend to sparkle. Heavy glass, because of the extra refraction tends to be more sparkly than normal light glasses.
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