Home » Rocks » Igneous Rocks » Obsidian
At 30 angstroms — a unit of measurement equal to one hundred millionth of a centimeter — an obsidian scalpel can rival diamond in the fineness of its edge. When you consider that most household razor blades are 300 to 600 angstroms, obsidian can still cut it with the sharpest materials nanotechnology can produce.2 Apr 2015. Obsidian on Specialty Laminate. Product photography and illustrations have been reproduced as accurately as print and web technologies permit. To ensure highest satisfaction, we suggest you view an actual sample from your dealer for best color, wood grain and finish representation.
What is Obsidian, How Does it Form, and What is it Used For?
Article by: Hobart M. King, PhD, RPG
- Black obsidian is a very powerful stone with a mysterious yet cleansing aura. It is commonly used for psychic protection, and it has the capacity to remove all the psychic smog that has been encapsulating its owner for quite some time.
- Check out our diamond obsidian selection for the very best in unique or custom, handmade pieces from our metaphysical crystals shops.
- Obsidian Minions are a type of Minion that collects Obsidian. 1 Description 2 Ideal Layout 3 Drops 4 Upgrades 5 Stats 6 Profits 7 Recipe Gallery Obsidian Minions are unlocked at Obsidian I Collection and can be placed on the Player's Island. The Obsidian Minion collects Obsidian. The Obsidian Minion can be crafted with a Wooden Pickaxe surrounded by 10 Obsidian in each field.
Obsidian: The specimen shown above is about two inches (five centimeters) across. The curved semi-concentric ridges are breakage marks associated with obsidian's conchoidal fracture. The rock has very sharp edges.
What is Obsidian?
Obsidian is an igneous rock that forms when molten rock material cools so rapidly that atoms are unable to arrange themselves into a crystalline structure. It is an amorphous material known as a 'mineraloid.' The result is a volcanic glass with a smooth uniform texture that breaks with a conchoidal fracture (see photo).
Where Does Obsidian Form?
Obsidian is usually an extrusive rock - one that solidifies above Earth's surface. However, it can form in a variety of cooling environments:
- along the edges of a lava flow (extrusive)
- along the edges of a volcanic dome (extrusive)
- around the edges of a sill or a dike (intrusive)
- where lava contacts water (extrusive)
- where lava cools while airborne (extrusive)
Types of Obsidian: The specimens shown above are from Glass Butte rockhounding site in central Oregon. It shows the diversity of obsidian types that can be found in a small geographic area. Clockwise from upper left are: double flow obsidian, rainbow obsidian, black obsidian, pumpkin obsidian, mahogany obsidian, gold sheen obsidian, and the piece in the center is gold sheen. The nice photo above is from the Glass Butte Rockhounding Site page on the Deschutes National Forest website.
Mahogany obsidian: A tumble-polished specimen of 'mahogany obsidian.' Image copyright iStockphoto / Arpad Benedek.
What Color is Obsidian?
|
Black is the most common color of obsidian. However, it can also be brown, tan, or green. Rarely, obsidian can be blue, red, orange, or yellow. The colors are thought to be caused mainly by trace elements or inclusions.
Occasionally two colors of obsidian will be swirled together in a single specimen. The most common color combination is black and brown obsidian swirled together - that's called 'mahogany obsidian' (see photo).
As a 'glass,' obsidian is chemically unstable. With the passage of time, some obsidian begins to crystallize. This process does not happen at a uniform rate throughout the rock. Instead it begins at various locations within the rock. At these locations, the crystallization process forms radial clusters of white or gray cristobalite crystals within the obsidian. When cut and polished, these specimens are referred to as 'snowflake obsidian' (see photos).
Rarely, obsidian has an iridescent or metallic 'sheen' caused by light reflecting from minute inclusions of mineral crystals, rock debris, or gas. These colored specimens are known as 'rainbow obsidian,' 'golden obsidian,' or 'silver obsidian,' depending upon the color of the sheen or iridescence. These specimens are very desirable for the manufacture of jewelry.
Snowflake obsidian: A tumble-polished specimen of 'snowflake obsidian.' Image copyright iStockphoto / Martin Novak.
What is the Composition of Obsidian?
Most obsidians have a composition similar to rhyolite and granite. Granites and rhyolites can form from the same magma as obsidian and are often geographically associated with the obsidian.
Rarely, volcanic glasses are found with a composition similar to basalt and gabbro. These glassy rocks are named 'tachylyte.'
Are There Other Glassy Igneous Rocks?
Pumice, scoria, and tachylyte are other volcanic glasses formed by rapid cooling. Pumice and scoria differ from obsidian by having abundant vesicles - cavities in the rock produced when gas bubbles were trapped in a solidifying melt. Tachylyte differs in composition - it has a composition similar to basalt and gabbro.
Obsidian outcrop: Obsidian along the edge of a lava flow in central Oregon. Image copyright iStockphoto / Phil Augustavo.
Obsidian knife blade: A knife blade manufactured from mahogany obsidian. The craftsman who made this blade had a very high skill level and was able to produce a serrated edge. Image copyright iStockphoto / Al Braunworth.
Occurrence of Obsidian
Obsidian is found in many locations worldwide. It is confined to areas of geologically recent volcanic activity. Obsidian older than a few million years is rare because the glassy rock is rapidly destroyed or altered by weathering, heat, or other processes.
Significant deposits of obsidian are found in Argentina, Canada, Chile, Ecuador, Greece, Guatemala, Hungary, Iceland, Indonesia, Italy, Japan, Kenya, Mexico, New Zealand, Peru, Russia, United States, and many other locations.
In the United States it is not found east of the Mississippi River, as there is no geologically recent volcanic activity there. In the western US it is found at many locations in Arizona, California, Idaho, Nevada, New Mexico, Oregon, Washington, and Wyoming. Most obsidian used in the jewelry trade is produced in the United States.
Obsidian spear point: A spear point fashioned from opaque black obsidian. Image copyright iStockphoto / Charles Butzin.
Rock & Mineral Kits: Get a rock, mineral, or fossil kit to learn more about Earth materials. The best way to learn about rocks is to have specimens available for testing and examination.
Uses of Obsidian as a Cutting Tool
The conchoidal fracture of obsidian causes it to break into pieces with curved surfaces. This type of fracturing can produce rock fragments with very sharp edges. These sharp fragments may have prompted the first use of obsidian by people.
The first use of obsidian by people probably occurred when a sharp piece of obsidian was used as a cutting tool. People then discovered how to skillfully break the obsidian to produce cutting tools in a variety of shapes. Obsidian was used to make knives, arrowheads, spear points, scrapers, and many other weapons and tools.
Once these discoveries were made, obsidian quickly became the raw material of preference for producing almost any sharp object. The easy-to-recognize rock became one of the first targets of organized 'mining.' It is probably a safe bet that all natural obsidian outcrops that are known today were discovered and utilized by ancient people.
Apache tears: 'Apache Tears' is a name used for small obsidian nodules of about one inch or less that can be found in volcanic areas of the southwestern United States. Their name comes from a Native American legend. During a battle between Apaches and the U.S. Cavalry in 1870, the outnumbered Apaches, facing defeat, rode their horses over a cliff rather than allow themselves to be killed by their enemy. Upon hearing the story of the battle, the tears of their family members turned to stone when they hit the ground. Those stones are now found as the black obsidian nodules. People who do rock tumbling often polish Apache Tears. They are difficult to polish because the obsidian chips and bruises easily. Success occurs when they are cushioned during the tumbling with smaller pieces of rough or small ceramic media.
Stone Age Manufacturing and Trade
The manufacture of obsidian tools by humans dates back to the Stone Age. At some locations, tons of obsidian flakes reveal the presence of ancient 'factories.' Some of these sites have enough waste debris to suggest that many people labored there for decades producing a variety of obsidian objects. Making arrowheads, spear points, knife blades, and scrapers from obsidian, chert, or flint might have been the world's first 'manufacturing industry.'
Obsidian was so valued for these uses that ancient people mined, transported, and traded obsidian and obsidian objects over distances of up to a thousand miles. Archaeologists have been able to document the geography of this trade by matching the characteristics of obsidian in outcrops with the characteristics of obsidian in cutting tools. A study done by the Idaho National Laboratory used composition studies by X-ray fluorescence to identify the source outcrops of obsidian artifacts and map their use across the western United States.
Obsidian in Modern Surgery
Although using a rock as a cutting tool might sound like 'stone age equipment,' obsidian continues to play an important role in modern surgery. Obsidian can be used to produce a cutting edge that is thinner and sharper than the best surgical steel. Today, thin blades of obsidian are placed in surgical scalpels used for some of the most precise surgery. In controlled studies, the performance of obsidian blades was equal to or superior to the performance of surgical steel.
Obsidian jewelry: Mahogany obsidian and snowflake obsidian cabochons set in sterling silver pendants.
Obsidian for opal triplets: A thin piece of obsidian is often used as a 'backing' material for opal doublets and triplets. The black obsidian adds stability to the opal and provides a dark background color that contrasts with the opal's fire.
Uses of Obsidian in Jewelry
Obsidian Vs White Diamond
Obsidian is a popular gemstone. It is often cut into beads and cabochons or used to manufacture tumbled stones. Obsidian is sometimes faceted and polished into highly reflective beads. Some transparent specimens are faceted to produce interesting gems.
The use of obsidian in jewelry can be limited by its durability. It has a hardness of about 5.5 which makes it easy to scratch. 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.
Other Uses of Obsidian
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 5.5 makes it relatively easy to carve. Artists have used obsidian to make masks, small sculptures, and figurines for thousands of years.
| More Rocks |
| Rock, Mineral and Fossil Collections. |
| Geodes |
| Hardness Picks |
| Fluorescent Minerals |
| Flint, Chert, and Jasper |
| Rock Salt |
| Lapis Lazuli |
| Tumbled Stones |
|
| ||
|
| ||
|
| ||
|
|
Home » Rocks » Igneous Rocks » Scoria
Article by: Hobart M. King, PhD, RPG
Scoria: A piece of scoria about 4 inches (10 centimeters) in diameter. Specimens with a rounded shape like this have usually been transported in a stream. This photograph was taken by Jonathan Zander and is used under a GNU Free Documentation License.
What is Scoria?
Scoria is a dark-colored igneous rock with abundant round bubble-like cavities known as vesicles. It ranges in color from black or dark gray to deep reddish brown. Scoria usually has a composition similar to basalt, but it can also have a composition similar to andesite.
Many people believe that small pieces of scoria look like the ash produced in a coal furnace. That has resulted in particles of scoria being called 'cinders' and the small volcanoes that erupt scoria to be called 'cinder cones.'
Obsidian Diamond Gold Ring
Scoria: The specimen shown is about two inches (five centimeters) across.
Obsidian Diamond
How Does Scoria Form?
Scoria forms when magma containing abundant dissolved gas flows from a volcano or is blown out during an eruption. As the molten rock emerges from the Earth, the pressure upon it is reduced and the dissolved gas starts to escape in the form of bubbles. If the molten rock solidifies before the gas has escaped, the bubbles become small rounded or elongated cavities in the rock. This dark-colored igneous rock with the trapped bubbles is known as scoria.
When some volcanoes erupt, a rush of gas blows out of the vent. This gas was once dissolved in the magma below. The gas often blows out small bodies of magma that solidify as they fly through the air. This action can produce a ground cover of scoria all around the volcanic vent, with the heaviest deposits on the downwind side.
Small particles of scoria that litter the landscape around the volcano are known as 'lapilli' if they are between 2 millimeters and 64 millimeters in size. Larger particles are known as 'blocks.'
Scoria cinder cone: Artistic drawing illustrating the subsurface magma source and layer-by-layer build-up of scoria in a cinder cone eruption. Image by USGS.
Sunset Crater cinder cone: Photograph of the Sunset Crater cinder cone that was formed by eruptions that occurred about 1000 years ago. It is located near Flagstaff, Arizona and is about 1000 feet tall. It is one of over 500 cinder cones in the San Francisco Volcanic Field. Image by USGS.
Cinder Cones
Most of the scoria falls to the ground near the vent to build up a cone-shaped hill called a 'cinder cone.' Cinder cones are generally small volcanoes produced by brief eruptions with a total vertical relief of less than a few thousand feet. They are usually very steep because scoria has an angle of repose of 30 to 40 degrees.
In some parts of the world, cinder cones occur in clusters of a few to hundreds of individual cones. These areas are called 'volcano fields.' An example of a volcano field is the San Francisco Peaks Volcanic Field near Flagstaff, Arizona, which contains over 500 cinder cones.
Stromboli ejecta: Magma being blown from the vent at Stromboli Volcano. This type of eruption would produce the small scoria cinders known as 'lapilli.' Photo by B. Chouet, USGS.
Lava Flows and Vesicular Basalts
Some newly erupted lava flows contain abundant dissolved gas. The gas bubbles in the flow move upwards towards the surface in an attempt to escape while the lava is still molten. However, once the lava starts to solidify, the bubbles are trapped in the rock. These trapped gas bubbles are known as vesicles.
If the upper portion of a lava flow contains a large concentration of vesicles, it is often called 'scoria' or 'vesicular basalt.' This material often has fewer vesicles and a higher specific gravity than the scoria of lapilli.
Scoria on Mars: This image shows a field on Mars that is strewn with pieces of scoria, erupted from a Martian volcano. The piece of Martian scoria in the foreground is about 18 inches across and was found on the surface of Mars by the Spirit Rover. NASA image.
The Beverage Bottle Analogy
Have you ever slowly opened a bottle that contains a carbonated beverage and watched the gas bubbles form on the walls of the bottle? Then as the seal on the bottle is broken, the bubbles grow larger and a hiss of gas escapes from the bottle, followed by a rush of foam. The depressurization and the escape of gas from a beverage is the same process that occurs when magma is depressurized as it emerges from a volcanic vent. The foam is equivalent to what will become scoria on solidification.
Mauna Kea cinder cone: A red cinder cone and a cinder-covered landscape at Mauna Kea, Hawaii. Photo by Scot Izuka, USGS.
Not to be Confused with Pumice
A vesicular igneous rock that is very similar to scoria is pumice. There are a few differences that can be used to distinguish them. First is their color. Scoria is almost always black or dark gray to reddish brown, while pumice is almost always white to light gray to light tan. This color difference is a result of their composition. Scoria forms from basaltic magmas, while pumice forms from rhyolitic magmas - which usually contain more gas.
Pumice has a much higher concentration of trapped bubbles - so many that the walls between them are very thin. The vesicles in pumice contain enough air that the rock will float on water. The thick walls of scoria make it heavy enough to sink.
Finally, when observed closely with a hand lens, you can often see tiny mineral crystals in scoria. However, close observation of pumice reveals a 'glassy' texture similar to obsidian. Pumice consists mainly of glass materials rather than mineral crystals. A 'glass' is a noncrystalline substance. In the case of pumice, it cooled so quickly that the atoms were unable to arrange themselves into ordered crystal structures.
Expanded aggregate: Photograph of 'light expanded clay aggregate,' a scoria look-alike that is produced by heating certain types of clay in a rotating kiln. Organic material and moisture in the clay produce gas that causes vesicles similar to those found in scoria. Straight from the kiln, the material has a smooth exterior, but when broken the vesicular structure is exposed. Expanded aggregate is used as landscape stone, lightweight concrete, lightweight fill, and as a substrate for hydroculture. Public domain image by Leca67.
Uses of Scoria
One of the main uses of scoria is in the production of lightweight aggregate. The scoria is crushed to desired sizes and sold for a variety of uses.
Concrete made with scoria typically weighs about 100 pounds per cubic foot. This is a weight savings compared to concrete made with typical sand and gravel that weighs about 150 pounds per cubic foot. This savings in weight allows buildings to be constructed with less structural steel. The air trapped in the scoria makes the lightweight concrete a better insulator. Buildings constructed with this lightweight concrete can have lower heating and cooling costs.
Crushed scoria is used as roofing granules, ground cover in landscape projects, and as a substrate in hydroponic gardening. Many dealers offer customers the option of choosing between black, brown, or red material. Scoria is also used as rip-rap, drainage stone, and low-quality road metal. Small amounts of scoria are used as sauna rock and as a heat sink in barbecue grills.
Rock & Mineral Kits: Get a rock, mineral, or fossil kit to learn more about Earth materials. The best way to learn about rocks is to have specimens available for testing and examination.
Scoria Substitutes
Where scoria is not available, a lightweight aggregate can be produced by heating shale in a rotating kiln under controlled conditions. With the proper type of shale, the material will have the properties, appearance, and vesicles of scoria. It is sold under the name 'expanded aggregate,' 'expanded clay,' or 'grow rocks' and used for the same purposes as crushed scoria.
Author's note: 'I am usually a cheerleader for the mining industry, but I will admit that it troubled me to see a cinder cone being removed to produce aggregate.'
| More Rocks |
| Rock, Mineral and Fossil Collections. |
| Geodes |
| Hardness Picks |
| Fluorescent Minerals |
| Flint, Chert, and Jasper |
| Rock Salt |
| Lapis Lazuli |
| Tumbled Stones |
|
| ||
|
| ||
|
| ||
|
|