In the early days glass tended to be coloured - a slightly brownish colour for most ordinary soda glass and a very pale grey for crystal. Crystal was achieved by adding lead salts to the mix which decolourised the brownness and also had the effects of increasing the refraction, decreasing the hardness(and thus making cutting easier), and of course, increasing the density. Modern glass is all clear unless it is ultra cheap of deliberately tinted.
Most modern crystal is what is called 24% lead content - typical for "Bohemia" crystal. The really high end tends to be even higher as the refraction is increased and the cutting becomes more brilliant. Thus Stuart always advertised 30% and Waterford may have been even higher. You should also mbe aware there are crystals at around 18%in the marketplace. Also "Kristal" can simply refer to glass.
There is a downside to the lead content. If the wine alcohol oxidises it first becomes acetone and the acetic acid(AKA vinegar). As it happens lead acetate is extremely soluble so what tends to happen is the a small amount of lead is leached out of the glass into the wine. This takes a long time but it leads to that milkiness common in the bases of old decanters.