The use of obsidian in jewelry can be limited by its durability. It has a hardness of about 5. It also lacks toughness and is easily broken or chipped upon impact. These durability concerns make obsidian an inappropriate stone for rings and bracelets. It is best suited for use in low-impact pieces such as earrings, brooches, and pendants. Obsidian is also used in making opal doublets and opal triplets. Thin slices or chips of opal are glued to a thin slice of obsidian to make a composite stone.
The black obsidian provides an inexpensive and color-contrasting background that makes opal's colorful fire much more obvious. It also adds mass and stability to the opal that facilitates cutting it into a gem. Freshly broken pieces of obsidian have a very high luster. Ancient people noticed that they could see a reflection in obsidian and used it as a mirror.
Later, pieces of obsidian were ground flat and highly polished to improve their reflective abilities. Obsidian's hardness of 5. Artists have used obsidian to make masks, small sculptures, and figurines for thousands of years. Article by: Hobart M. Find Other Topics on Geology. Maps Volcanoes World Maps. Rainbow Obsidian: A baroque cabochon of iridescent "rainbow obsidian.
Rock, Mineral and Fossil Collections. Hardness Picks. Flint, Chert, and Jasper. Tumbled Stones. Fluorescent Minerals. Lapis Lazuli. Rocks: Galleries of igneous, sedimentary and metamorphic rock photos with descriptions. Minerals: Information about ore minerals, gem materials and rock-forming minerals. Volcanoes: Articles about volcanoes, volcanic hazards and eruptions past and present. How does crystal size relate to how long the rock took to cool?
What is obsidian? What are some types of intrusive igneous rocks? What is rhyolite? So, what does the heat do to the rock?
It bakes the rock. Remember, all rocks are made up of mineral crystals, or pieces of other rocks made up of crystals. Baked rock does not melt, but it does change. It forms crystals. If it has crystals already, it forms larger crystals. Because this rock changes, it is called metamorphic. Remember that a caterpillar changes to become a butterfly.
That change is called metamorphosis. Metamorphosis can occur in rock when they are heated to to degrees Celsius. When Earth's tectonic plates move around, they produce heat. When they collide, they build mountains and metamorphose the rock. The rock cycle continues. Mountains made of metamorphic rocks can be broken up and washed away by streams.
New sediments from these mountains can make new sedimentary rock. The rock cycle never stops. Igneous Rocks : Igneous rocks are a type of rock formed from extremely hot 2, degrees F molten masses known as magma. Generally, magma lies about 90 miles below the surface.
In certain places, such as Yellowstone National Park, the magma is as close as 40 miles below the surface. On average, every feet you dig down into the earth, the temperature will increase about 1.
Sometimes magma forces its way up to the surface through a vent such as a volcano and spills onto the surface.
This happened near Flagstaff, Arizona at Sunset Crater less than 1, years ago. Once magma comes out onto the surface of the earth it is called lava, and it cools rapidly at the surface. Extrusive igneous rocks can be distinguished by their small crystal sizes. In all extrusive igneous rock, it is nearly impossible to detect crystals without the aid of a microscope. The faster the rock cools, the smaller the crystals. Some extrusive igneous rocks cool so quickly that they have a glassy texture.
Common extrusive igneous rocks are: andesite, basalt, dacite, pumice, rhyolite, and obsidian. Intrusive igneous rocks solidify over a period of thousands of years. This slow cooling rate allows better development of mineral crystals. Intrusive igneous rocks will have eye visible crystals and will appear coarse-grained.
Igneous intrusive rocks eventually will become exposed at the surface of the earth by erosion of the overlying material. Common intrusive igneous rocks are: granite, diorite, gabbro, and peridotite. The La Sal Mountains were formed by widespread igneous activity that began about 40 million years ago. Caldera explosions erupted thousands of cubic miles of volcanic rocks from several locations. Volcanoes spewed ash and lava. For 20 million years these extrusive volcanic rocks smoothed the landscape, filling depressions with accumulations of ash, flows, and debris literally miles thick.
These mostly pastel-colored extrusive rocks still blanket much of the high areas of central and southwestern Utah. Not all of the molten rising igneous material erupted as volcanic rocks; some material, along with its mineral-bearing fluids, congealed in the earth's crust.
Several of these intruded masses having been exposed by erosion or encountered out by exploration drilling became great mining districts, such as at Alta, Brighton, Bingham, Park City, and Cedar City. In the Colorado Plateau, bodies of intrusive rocks domed the overlying sedimentary rocks to form the Abajo and Henry Mountains as well as the La Sal Mountains. NOTE: This paragraph on laccoliths borrowed from here. Rocks which have undergone these sorts of changes are called metamorphic rocks.
Agents of Matamorphosis. The three primary agents which metamorphose rock are temperature, pressure, and fluids. High temperatures can change rock by changing the structure of the minerals which make up the rocks; changing the structure of the minerals changes them into new minerals remember the definition of a mineral.
This increase in temperature with increase in depth is called the geotherm. Intrusions Another source of high temperatures inside the Earth is magma intruding cooler rock. These temperature increases are localized near the intrusion, but also metamorphose rock this is called contact metamorphism. Pressure or more properly, stress can also change rock. There are two main kinds I want you to know about: Confining Pressure Pressure due to the weight of overlying rock.
When a rock with flat or elongated minerals is put under immense pressure, the minerals line up in layers, creating foliation. Foliation is the aligning of elongated or platy minerals, like hornblende or mica, perpendicular to the direction of pressure that is applied.
An example of this transformation can be seen with granite, an igneous rock. Granite contains long and platy minerals that are not initially aligned, but when enough pressure is added, those minerals shift to all point in the same direction while getting squeezed into flat sheets. Nonfoliated rocks are formed the same way, but they do not contain the minerals that tend to line up under pressure and thus do not have the layered appearance of foliated rocks.
Sedimentary rocks like bituminous coal, limestone, and sandstone, given enough heat and pressure, can turn into nonfoliated metamorphic rocks like anthracite coal, marble, and quartzite. Nonfoliated rocks can also form by metamorphism, which happens when magma comes in contact with the surrounding rock. Igneous Rocks Igneous rocks derived from the Latin word for fire are formed when molten hot material cools and solidifies. Igneous rocks can also be made a couple of different ways.
When they are formed inside of the earth, they are called intrusive, or plutonic, igneous rocks. Granite and diorite are examples of common intrusive rocks. They have a coarse texture with large mineral grains, indicating that they spent thousands or millions of years cooling down inside the earth, a time course that allowed large mineral crystals to grow.
Alternatively, rocks like basalt and obsidian have very small grains and a relatively fine texture. This happens because when magma erupts into lava, it cools more quickly than it would if it stayed inside the earth, giving crystals less time to form. Obsidian cools into volcanic glass so quickly when ejected that the grains are impossible to see with the naked eye. This happens when the ejected magma still has gases inside of it so when it cools, the gas bubbles are trapped and end up giving the rock a bubbly texture.
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