 Hi I'm Maureen Feynman and I'm Heather McHugh Brune. In this module we're going to be talking about marble. So marble is a metamorphic rock that started its life as a sedimentary rock. So in its early days when it was young it started out as a limestone. Limestone being one of the most common sedimentary rocks on the surface of the earth. We have a lot of it right here around us. You can't garden without finding it. It's everywhere here. Both limestone and marble are made almost entirely of the mineral calcite. So we talked about calcite a little bit in the alabaster module. Here we're going to be looking at it in a case of marble being composed almost entirely of calcite. Really neat mineral. I have a nice specimen of it here. So you can see again how that crystalline structure, in this case we're in the hexagonal crystal system, stacks upon and upon itself to give us these nice rhombic shaped crystals that cleave perfectly along three different axes or three different crystal dimensions. One of the really cool things about calcite, there are a lot of cool things about calcite, but one of the really cool things about calcite is that it has a very high birefringence. So what that means is that light when it enters into the crystal actually moves in different velocities in different directions within the crystal structure. And what happens when you've got light traveling at different speeds in different directions is that a beam of light entering the crystal is split into two beams. One is what we call an ordinary beam and the other an extraordinary beam. So I'm going to show you, this is a, I hope you'll be able to see this, a laser pointer, one single beam of red light. When I shine this in the back here, that single beam gets split into two and you should see it emerging on the other side as two dots of light or two points of light. One of those is passing directly through the crystal and the other one is getting shifted off at an angle. This also gives you a property where you can see a double image when you look through a crystal. Again, that double image coming from the one ordinary ray of light and the other extraordinary ray of light that gets bent off at an angle. And that is really just due to the way the atoms are bonded in the crystalline structure at the tiniest, tiniest structural level within these crystals. So once this limestone made up of calcium carbonate is subjected to high pressures and temperatures inside of the earth, for example, in a mountain building situation where you have two plates that are pushing into each other and pushing up mountains, the calcium carbonate simply recrystallizes to form a larger, more nicely shaped crystals. And that gives you very often what we call a sugary texture in the, in the well developed marble, very in the case of a very pure calcium carbonate will give you a very white, you can see, so this beautiful pure white, very sugary marble here. This one has a little bit of this gray streaking through it is also pretty common in marbles that gray is actually graphite. So the calcite being calcium carbonate, the graphite being simply carbon and that can be either leftover organic carbon from its sedimentary days, or that can be carbon that's being reduced out of the calcium carbonate by deprivation or by having the oxygen removed from the from the carbonate there. So we have a soft mineral calcite with a hardness three, a little bit of graphite in there with it with a hardness of one all carbon carbon based coming together to make either in its purest case that very snow white marble or in some cases the beautiful silver streaked marble with graphite in it. So what makes marble so important for art history and that's that's kind of a difficult thing to say because what doesn't make marble important for art history. I think hopefully most people when they think of rocks and art should think immediately of marble. We have so many fine examples of works of art from the ancient world, the medieval world, the early modern period and even today made of marble and the karara quarries are still producing sculpture quality marble, there are other places where you can obtain it as well. And part of what makes this such a desirable material is that like alabaster it's fairly easy to work. It's not quite as easy as alabaster but you can get a fineness of detail and that crystalline structure means that you can play with texture in marble. You can leave things rough and have kind of a softness to them. You can polish them to a high sheen of and think of say a marble cutting board or a cheese board that you might have that tends to be heavily polished or this tile here that I got from an importer in Cleveland, Ohio. You can get different effects from different treatments of the surfaces and so you can imitate human flesh. You can imitate something hard like a shell and this is something that has been exploited by sculptors for centuries.