Crystal faces form only where there is enough open space in the rock mass to allow the natural geometric forms of the crystals to develop as free faces. When crystallization occurs, the atoms that comprise a mineral become arranged in regular, geometric patterns that are unique to the specific mineral. All rocks consist of mixtures of various crystalline minerals. When I say "crystals," don't visualize those beautiful pointed prisms of quartz found in geodes. This volcanic glass has an almost total absence of sizable mineral crystals within the glass matrix. Obsidian is natural glass that was originally molten magma associated with a volcano. My goal in this article is to increase your awareness of some of the more fascinating aspects of this incredible stone. The ancient volcanic hills called Glass Buttes hold a dazzling variety of gem-quality obsidian, including: mahogany, red, flame, midnight lace, jet black, pumpkin, brown, rainbow, gold sheen, silver sheen, green, lizard skin, snowflake and more. But until a few years ago when I made my first obsidian collecting trip to Glass Buttes, Oregon, I thought obsidian was pretty much just black glass. Sharp and shiny, obsidian is so different from other rocks. Ever since I had my first rock collection as a child, I've loved obsidian. Pumice is ground up and used today in soaps, abrasive cleansers, and also in polishes.Obsidian. Some lava blocks are large enough to carry small animals. Huge pumice blocks have been seen floating on the ocean after large eruptions. Pumice is so light that it actually floats on water. As the lava hurtles through the air it cools and the gases escape leaving the rock full of holes. The lava is ejected and shot through the air during an eruption. Pumice is formed from lava that is full of gas. Pumice is a very light colored, frothy volcanic rock. The minerals that make up rhyolite are quartz, feldspar, mica, and hornblende. Rhyolite is an extrusive igneous rock having cooled much more rapidly than granite, giving it a glassy appearance. These crystals are so small that they can not be seen by the naked eye. The difference is rhyolite has much finer crystals. Rhyolite is very closely related to granite. It is composed mostly of the mineral plagioclase feldspar with smaller amounts of pyroxene and olivine. Gabbro is very similar to basalt in its mineral make up. Gabbro is a dark-colored, coarse-grained intrusive igneous rock. Today obsidian is used as a scalpel by doctors in very sensitive eye operations. Obsidian is usually black or a very dark green, but it can also be found in an almost clear form.Īncient people throughout the world have used obsidian for arrowheads, knives, spearheads, and cutting tools of all kinds. Obsidian is produced in nature in a similar way. When people make glass they melt silica rocks like sand and quartz then cool it rapidly by placing it in water. The lava cools so quickly that no crystals can form. Obsidian is produced when lava cools very quickly. Notice in the photo to the left how it fractures. When obsidian breaks it fractures with a distinct conchoidal fracture. Obsidian is a very shiny natural volcanic glass. The principle minerals that make up dacite are plagioclase, quartz, pyroxene, or hornblende. The Canadian Shield of North America contains huge outcroppings (surface rocks) of granite.ĭacite is an extrusive igneous rock. The State of New Hampshire has the nickname "Granite State" because of the amount of granite in the mountains of that beautiful state. Granite is quarried in many places in the world including the United States. Engravings in the granite can be read for hundreds of years, making the rock more valuable. It is also used for monuments because it weathers slowly. Granite is an excellent material for building bridges and buildings because it can withstand thousands of pounds of pressure. Look at the photo of granite above, notice the different crystals in the rock. This allows crystals of the four minerals to grow large enough to be easily seen by the naked eye. Because it hardens deep underground it cools very slowly. Granite forms as magma cools far under the earth's surface. These minerals are quartz, feldspar, mica, and usually hornblende. Granite is an igneous rock that is composed of four minerals. Some basalts are intrusive having cooled inside the Earth's interior. Most basalts are volcanic in origin and were formed by the rapid cooling and hardening of the lava flows. They are the most widespread of all the igneous rocks. The mineral grains are so fine that they are impossible to distinguish with the naked eye or even a magnifying glass. Basalts are dark colored, fine-grained extrusive rock.
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