 In the first part of this module, we learned about the geology of igneous rocks, again coming back to looking at materials that form by the cooling of molten magma beneath the surface of the earth or a flowing lava at the surface of the earth. And as that liquid cools and crystallizes, we form crystalline rocks. As we've discussed before a little bit, Heather and I had a chance to exchange some terminology and develop a common vocabulary over the course of making this class. And one of the really interesting things that we discovered early on is that the way geologists use the term porphyry or porphyritic and the way art historians use the word porphyry, have two slightly different meanings and they're in no way mutually exclusive, but they do have a somewhat different significance. So we have a couple of examples of porphyry here, one taken from a riverbed and one nicely cut here. What makes this geologically speaking porphyry is actually the texture of the rock and the distribution of crystal grain sizes. So in this particular rock, I believe you can see that there are some lighter colored large feldspar crystals that have grown within the rock. And then those large crystals are contained within a fine grained matrix. So typically we define an igneous rock as being either coarse grained or fine grained and the porphyry is a little bit funny because it's a bit of each. And the way this works is that the feldspar's were growing slowly within a magma chamber beneath the surface of the earth. And then there was an eruption that carried the magma with those large feldspar crystals already in it up to the surface and then proceeded to cool the rest of the way very quickly. So we have some large light colored crystals that reflect the slow early growth beneath the surface and then some darker very fine grained crystals that reflect the final stage of growth upon eruption. Not always, but in some cases the oxidation of iron in the matrix of this igneous rock, which means in the lava that it crystallized from, serves the purpose of coloring it a beautiful purple color. And that's where the meaning of porphyry particularly in the context of art history comes in. If you were to ask an art historian generally about porphyry, probably the picture that would pop into the art historian's head would be of a purplish reddish porphyry like this. And that's because some of the most famous works made from porphyry are made from what we call imperial porphyry. And it is this very rare purplish color that in the ancient world could come only from a single quarry in Egypt in the desert mountains between the Nile and the Red Sea. And there was tremendous effort just to get to this single quarry. You had to have caravans, you had to have guards, you had to have water, all sorts of difficulties. And they finally abandoned it because it was such a difficult thing to obtain. But this rare stone was reserved just for members of the imperial family. And just like other forms of purple that were quite rare like Tyrion purple, a dye that is derived from thousands and thousands and thousands of sea snails. They have to give up their little gallbladders and it takes like 10,000 of them to do enough to trim a Roman toga. This purple was rare and difficult to obtain and therefore it was incredibly high status. And it leads to words like porphyrogenitos which in Greek means born to the purple. That's how you describe someone who is of the imperial family.