Obsidian Rock – Meaning, Uses, Facts, Properties & Color
Mar 27, 2023
We know three main classes of rocks, sedimentary, igneous, and metamorphic. In the Limestone article, we learned how sedimentation occurs and the formation of sedimentary rocks takes place. Today, we are going to know Obsidian rock, which falls in the main class of igneous rocks.
We know igneous rocks have two basic types:
1. Intrusive Igneous Rocks
When molten lava/magma is cooling slowly below the surface of the earth and forms large crystals in the rocks, they are known as intrusive igneous rocks. For example-
2. Extrusive Igneous Rocks
When molten magma is cooling rapidly onto the surface of the earth and forms small crystals in the rocks, they are known as extrusive igneous rocks. For example-
- Fire Opal
What Is Obsidian Rock?
Obsidian is a glassy igneous rock that comes in a wide range of colours and textures. It forms when felsic lava extruded from a volcano onto the surface of the earth.
The cooling of lava takes place so rapidly that no chance remains for proper crystallization (Atomic Arrangements) and finds as an amorphous glass material. It has a smooth and uniform texture, which breaks with a conchoidal fracture.
Geological Classification of Obsidian Rocks:
Let’s see the classification of obsidian rocks from the perspective of a geologist.
- Class: Igneous Rocks Sub-Class: Medium Hardness Rocks
- Family-Group: Volcanic
- Category: Opaque Rocks
- Alternate Names: Lava, Glassy Lava, Xaga, Royal Agate, Glass Agate, Volcanic Glass, Glass Gem, Guardian Gem, and Black Beauty
Formation of Obsidian Rocks in Nature
We have seen in the earlier section that obsidian is a type of extrusive rock. It forms on the surface of the earth when magma or lava comes out due to volcanic phenomena and rapid cooling takes place. Obsidian rock can form in a variety of cooling environments such as-
- Along the edges of a flow of magma or lava
- Along the edges of a dome of the volcano
- Along the edges of a sill or dike, which takes place under the surface and it is the only intrusive type
- The point where magma comes into contact with water on the surface
- The point where magma cools due to airborne cooling effects
Properties of Obsidian
Obsidians have various physical & visual properties, such as Hardness, Grain Size, Fracture, Streak (Colors), Porosity, Luster, and Strength. These all property attributes determine the applications of obsidian rocks. Let’s see some significant properties of obsidians.
Specific Gravity & Density of Obsidian
The density of rock is used to express specific gravity. It is measured in relevance to the density of water in gram per cubic centimeter of the mass. The density of obsidian is 2.55 g/cubic cm. It means it is heavier than water and dense medium as a mass.
Streaks & Colors of Obsidian
The streak of obsidian is white, but various colors occur in nature.
Frequently found colors of Obsidian rocks are:
The highest occurrence of obsidians is in Black, but it is also available in Brown, Green, or Tan.
Rarely found colors of Obsidian rocks are:
A very rare occurrence of obsidians is Blue, Orange, Red, and Yellow.
The occurrence of color is due to the presence of different trace elements in obsidian rocks.
Compressive Strength of Obsidian
When a compressive pressure (In Newton unit) is applied to the rocks, a permanent deformation of rocks takes place at a point of pressure. It is termed as the compressive strength of rocks and obsidians fractured or permanently deformed at 0.15 N per square millimeter. It means obsidian is easy to break and chipped by some impacts. It also renders obsidian the unfit stone for rings and bracelets like jewelry production.
Obsidians break into the conchoidal fracture. Obsidian has no natural or defined planes of separation when a pressure exceeding its physical strength exerts on it. Therefore, smoothly curved fracture takes place on the surface of obsidian glass rock.
The images at the side depict the phenomena beautifully and precisely.
The hardness of Obsidian (How Hard Is Obsidian Rocks)
In physics and geology, the hardness of the physical objects or rocks is measured by Moh’s scale, which rates the objects in question on a scale from 1 to 10.
- Rocks with Moh’s hardness scale from 1 to 3 are considered soft rocks.
- From 3 to 6 scale – medium hardness
- From 6 to 10 hardness scale – the hardest rocks
Based on Moh’s hardness scale, obsidians fall into a medium-hard category of rocks because obsidian rocks have 5-5.5 Moh’s hardness scale. It also means obsidian is easy to scratch.
Porosity of Obsidian
It is a less porous rock, so it has a medium hardness.
Luster & Transparency of Obsidian
In Obsidian, only 1% of water exists. It means it is vitreous, and due to the high presence of silica and other glass-forming minerals, it is translucent in transparency.
What Type of Rock Is Obsidian?
Geologically, rocks are used to divide into types. Based on the texture of the Obsidian rock, types are decided. The main types of Obsidian rocks are as follows:
When two colors, Black and Brown, swirled together in a single obsidian rock, Mahogany Obsidian takes place.
Obsidian is kind of glass and chemically unstable. Overtime crystallization process takes place in the mass of rock. If the formation of crystals happens in a radial cluster of white or gray crystals (cristobalite) at different and isolated locations, it is referred to as Snowflake obsidian.
Seldom obsidians have an iridescent or metallic “Sheen” thanks to the reflection of light rays. It used to take place due to minute inclusions of mineral crystals, gas trapped in obsidian, and some debris. It results in a prismatic effect found due to the glass matter of obsidians, and white light rays are fragmented in the rainbow color spectrum.
Gold Sheen Obsidian
If metallic sheen reflects light rays in golden color, the gold sheen obsidians occur. In black obsidian, golden inclusions take place. It shines as yellow and honey metals in the mass of obsidian.
Silver Sheen Obsidian
Similarly, if the metallic sheen is found in silver color, the silver sheen obsidians take place.
Purple Sheen Obsidian
When tiny bubbles of gas are trapped in the rock and result in a reflection of purple color, a purple sheen obsidian happens.
Double Flow Obsidian
Sometimes, the two different colors exist in the same specimen of the obsidian rock.
Moreover, the occurrence of different colors takes place in a pattern of flows, and it is flowing in a different direction, we termed it a double flow obsidian.
Black is the most common color found in obsidians. Therefore, the highest occurrence of obsidians was found in the black glass. It projects a shiny black hue and is popular as a black gemstone in the jewelry industry.
When obsidian reflects mainly pumpkin color in pale brown shades, it is called pumpkin obsidian. It resembles Mahogany obsidians in looks and brown to black and beige bands of colors found in the mass of obsidians.
Green-colored obsidians occur seldom, and when polished, they resemble emeralds and are used for jewelry purposes. It is found in mossy to pickle green color shades.
The occurrence of Obsidian Rocks in Different Geographic Regions of the Earth
Obsidian, falls into a volcanic group of rocks. Therefore, the occurrence of Obsidian is confined to geological areas where volcanic activity happened in the recent past. Obsidian rocks are not expected in volcanic areas of distance past billions of years because the glassy rocks are destroying rapidly due to weathering effects and various geological processes.
Significant deposits of obsidian rocks exist in the following continents of the world.
Indonesia, Japan, Armenia, Azerbaijan, and Russia, Afghanistan.
Greece, Hungary, Iceland, Italy, Scotland, and Turkey.
Canada, Mexico, and the USA.
El Salvador, Guatemala, and Papua New Guinea.
Argentina, Chile, Ecuador, and Peru.
The highest production of jewelry grade obsidians takes place in the United States. However, in the USA, obsidian rocks are not found in eastern regions of the Mississippi River because that area lacks volcanic activities in the recent past. The remaining areas of the USA where obsidian rocks are available:
Obsidian rocks are also available in Virginia, Pennsylvania, and North Carolina.
Now, I am going to acknowledge your use of obsidian. Do you know a conchoidal fracture of a rock resulting in pieces of stones on curved surfaces?
Obsidian rocks can produce fragments with sharp edges, and sharp fragments of obsidian rocks were first used as cutting tools in ancient eras of the civilization of humankind.
Today, a variety of uses are found in different industries, such as
Obsidians as Weapons & Tools:
The human civilization has used obsidian rocks to make knives, spear points, arrowheads, scrapers, and various weapons.
Knives and blades were fine for kitchen applications. Scrappers were used in land digging, agriculture, and gardening applications. Arrowheads and spear points were provided the sharpest tools for warriors in the creation of various weapons.
Obsidians in Construction Industry:
Obsidian rocks use in interior and exterior decoration. Decorative aggregates, gravels, and pebbles from obsidian rocks are a favorite material for creating architectural beauty and artwork on walls, ceilings, and floors of the buildings. Artworks in gardens, patios, swimming pools, fire features, and water features look amazing when obsidian rocks are applied along with other Gemstones.
It is also used as mirror elements in the decoration of the interior, exterior, and construction of artifacts for various applications in life.
Obsidians in Medical Surgery:
Obsidian rocks are continuously used in the modern era to create cutting-edge surgical tools. Studies have revealed the fact that thin blades for scalpel of obsidians are superior in sharpness and performance to or equivalent to blades created from the latest surgical steel.
Do you know household razor blades are 300-600 Angstroms while obsidian can produce blades of 30 Angstroms? It means obsidian can produce the sharpest materials that only Nanotechnologies can produce.
If you look at the sharpest edge of the steel blade in the microscope, you will notice almost saw-like teeth on the edge. At the same time, the obsidian blade has no bite because it has an evenly smooth edge at the microscopic level. Therefore, wounds cut by obsidian blades heal significantly faster than wounds by steel blades.
Obsidians in Antiquity and Jewelry Industry:
Obsidian rocks are often cut into beads and cabochons. Mahogany and Snowflakes obsidian types of rocks provide gemstones for earrings, brooches, and pendants.
In the creation of Opal Doublets and Triplets, a thin slice of obsidian is glued as a backing material to construct composite gemstones. Black obsidian is an ideal material for making Opals for offering color-contrasting background.
However, the medium strength of obsidian with 5.5 hardness creates durability concerns, and the jewelry industry avoids its use as they apply diamonds and other gemstones.
Obsidian Rocks for Sale
Obsidians are soft rocks for many jewelry applications where the chances of various impacts are high. So, the jewelry industry avoids the increased usage of these natural rocks/gems/glasses. It is mainly used in delicate jewelry, and the following varieties are famous for commercial usage of obsidians.
The wealthiest market of obsidian for sales is aimed at healing purposes. It is believed that obsidian stones have natural healing capabilities. Therefore, along with described various obsidian types earlier, the following varieties of obsidian products are in demand in the healing gemstone market.
- Apache Tears
- Cat Eye
- Midnight Lace
Wrapping It Up
In the series of identifying various rocks and natural stones at World of Stones, USA, we have explored different aspects of obsidian rocks such as meaning, uses, facts, properties, and colors of obsidian edge Stones, USA is an excellent place to buy a variety of natural rocks used primarily for exterior applications in the building construction industry. If you are interested to know about rocks and stones, keep reading blogs at World of Stones.