 In the first half of this module, we learned about the physical and chemical properties of rock crystal, the mineral quartz. Again, the chemical composition being SiO2 and the rock crystal, very clear, very colorless is a particularly pure form of SiO2 in crystalline form. The addition of slight amounts of impurities such as metals can lend different colors to the crystal, like the amethyst that you see here. And we're gonna move on now in the second half of the lesson of this module into learning about how these materials were used in art history, which is Heather's expertise. So in the art history lesson, one of the first things we're gonna be talking about is acquiring and working with rock crystal. And even though rock crystal was relative, quartz is found everywhere and is relatively common, really beautifully clear and pure rock crystal without a whole lot of imperfections is rather rare. And not only was it rare, but it was somewhat difficult to work. It tends to be both hard and brittle. And so it has to be worked very, very carefully if you strike it percussively, you run the risk of some disaster and the destruction of your piece. And although we know that this could be worked percussively, it had to be done with a great deal of skill. And we're going to see a series of videos in which an experimental archeologist, Elise Moreiro describes the actual process for creating particular types of vessels out of rock crystal. And as we'll see as we look at tools and images from history and read descriptions, these technologies are ones that existed at least from the ancient Egyptians up close to the present day. And in some areas of India where they're still doing gem engravings and carvings without electric tools, you can still see evidence of that same technology today. Then we'll shift to a discussion of symbolic properties of rock crystal and the ways in which this association with purity, with water and with ice were picked up and given different religious symbolism in different periods. And of course, then we'll look at a series of case studies.