 In this module, we've learned about rock crystal. So rock crystal is a version or a variety of quartz, which is the most common mineral on the surface of the earth. Quartz takes a variety of different forms and shapes and colors, depending on the impurities and inclusions that are found within it. In the case of rock crystal, however, what we're dealing with is the most pure, clear, unadorned, unaccessorized version of the mineral quartz and the physical properties, the clarity, the hardness, the transmission of light, all come back to reflect that pure, clean SIO to silicon and oxygen only in a perfectly crystallized structure to form our rock crystal mineral. And as we've seen in our case studies, a whole number of really extraordinary objects were made from rock crystal. These were very difficult objects to make because rock crystal is both hard and brittle. It was a tough material to work with, so it required a degree of expertise on the part of the artist that would have meant that these would be high status, high price works of art. And I hope that the case studies and also the videos with Elise Moreiro describing techniques have helped you to appreciate some of the difficulty value in working with this medium and also consider how difficult it is to find large scale specimens that are truly clear and unblemished. So that rarity also adds to price. But then we also saw how these artists enhanced those properties of rock crystal, taking advantage of its ability to magnify, to shine and reflect light. And this symbolic association throughout history, really, with purity and clarity.