Rocks

Igneus Rock: Obsidian. 

Produced when lava cools rapidly and with minimum crystal growth. Obsidian is commonly found where the chemical composition of lava induces a high viscosity. Since it is hard and brittle it easily fractures and can have very sharp edges. These sharp edges have been used in the past as tools for cutting and piercing and as surgical scalpel blades. Two interesting nicknames for Obsidian that i found are "Apache Tears" and "Volcano Glass". I chose it because of its unique experimental uses and because it is so beautiful. http://www.mindat.org/min-8519.html


Metamorphic Rock: Marble 

Marble. 

Marble is formed by the metamorphing of sedimentary rocks, mostly limestone. It is often used in sculpting and building because it is a soft rock, easily shaped and molded, and also because it is so beautiful. Marble is famous for its characteristic many colors, as well as its colored swirls and veins which are usually due to mineral impurities. I chose it because it is so commonly used to make statues and in homes, making it a highly relevant rock. (We actually wanted to use marble in our house, to build our kitchen counters, but chose a marble-granite mix instead because marble is so soft that we would damage it with kitchen wear-and-tear) http://www.softschools.com/facts/rocks/marble_facts/371

Sedimentary Rock: Sandstone

                                                                    Sandstone. 

Sandstone is a classic sedimentary rock made up of sand-sized stone granules. It is commonly compose of Quartz and Feldspar and is a very porous rock. Some types of sandstone are excellent materials from which to make grindstones, for sharpening blades and making grindstones for grinding grain. I chose them because of their many uses and because it is so colorful and emblematic of the American Western landscape. http://geology.about.com/od/more_sedrocks/a/aboutsandstone.htm