 Hi, I'm Maureen Feynman. And I'm Heather McKeown-Brune. In this week's module we're going to be talking about rock crystal or the mineral that geologists commonly refer to as quartz. So first of all, let's start with a couple of definitions. What is a crystal? So a crystal is defined as a repeating structure of atoms that it's repeated over and over again in order to create an internal structure to a given material. When you hear the word crystal, you might immediately be drawn to thinking about things like this glass that go on your table for things like drinking and filling with flowers and jelly dishes and such. This is when we say put the crystal on the table, typically what we're referring to. This crystal has sort of an interesting story to it because what it is is indeed not a crystal at all. This is a form of leaded glass. And glass is a material that is made primarily out of silicon and oxygen, the same that quartz is, but is different in that it is what we call an amorphous material, amorphous meaning that it lacks any sort of internal structure like a repeating pattern of atoms that would give it the definition of a crystal. So this crystal is not a crystal, but we call it crystal because it was initially developed in order to replicate some very rare and expensive pieces of glassware, so to speak, that were carved from what we call rock crystal. Now rock crystal, like I have in my hand here, is indeed a crystal. It's the mineral quartz SiO2. This is one form of SiO2 in the crystalline structure here. And quartz can take on a number of different colors and clarities and optical properties, but when it has this nice pure water clear color to it, we call it rock crystal. And it was this rock crystal that was used for carvings and vessels in antiquity as Heather can tell you about. Well, in the antique period, the Greeks and Romans believed that rock crystal like this was actually an incredibly pure form of ice, ice that had formed in the heavens. And because they associated it with ice, they particularly favored drinking vessels for cold liquids and would not put hot liquids into their rock crystal. And they also noted its optical qualities. They would often make vessels that looked like bunches of grapes, and then when they filled them with wine, the color, this clear color of the crystal would take on the color of its contents. And so that was one of a number of properties that were particularly favored. And we're going to explore those properties when we get to the art history portion.