Obsidian: The specimen shown above is approximately two inches (five centimeters) in diameter. Curved semi-concentric ridges are traces of fractures associated with rupture of the obsidian pinna. The rock has very sharp edges.
What is Obsidian?
Obsidian is an igneous rock that is formed when molten rock material cools down so quickly that the atoms are unable to form a crystalline structure. It is an amorphous material known as a “mineraloid”. The result is volcanic glass with a smooth uniform texture that breaks with a shell fracture (see photo).
Obsidian is typically an extruding rock – one that solidifies above the Earth’s surface. However, it can form in various cooling environments:
- along the edge of the lava flow (extrusion)
- along the edge of the volcanic dome (extrusion)
- around the edge of a window sill or dike (intrusive)
- where the lava meets the water (extrusion)
- where the lava cools in the air (extrusion)
Types of Obsidian: The specimens shown above are from the Glass Butte exploration site in central Oregon. It shows the variety of types of obsidian that can be found in a small geographic area. Clockwise from the top left are Dual Flow Obsidian, Rainbow Obsidian, Black Obsidian, Pumpkin Obsidian, Mahogany Obsidian, Golden Gloss Obsidian, and the center piece has a golden sheen. The nice photo above is from the Glass Butte Rockhounding Site on the Deschutes National Forest website.
Mahogany Obsidian: A 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, light brown or green. Rarely, obsidian can be blue, red, orange, or yellow. The colors are thought to be mainly caused by trace elements or inclusions.
Sometimes the two colors of obsidian are mixed together in a single copy. The most common color combination is black and brown obsidian twisted together – this is called “mahogany obsidian” (see photo).
As “glass”, obsidian is chemically unstable. Some obsidian begins to crystallize over time. This process is not happening at the same pace throughout the scale. Instead, it starts at different places in the rock. At these points, the crystallization process creates 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 off tiny inclusions of mineral crystals, rock debris, or gas. These colorful specimens are known as “rainbow obsidian”, “golden obsidian” or “silver obsidian”, depending on the color of gloss or iridescence. These specimens are in great demand for jewelry making.
Snowflake obsidian: a polished specimen of “snowflake obsidian”. Image copyright iStockphoto / Martin Novak.
Volcanic glass and obsidian are therefore not synonymous, although both terms can often be freely used. You certainly don’t use “volcanic glass” in place of “obsidian”, but be careful – volcanic glass is not always obsidian.
How Obsidian is Formed?
Obsidian is formed when a volcano’s felted lava cools quickly with minimal crystal growth. Chemicals (sodium silica content) cause increased viscosity, which forms natural lava glass when it dries quickly. Chemical composition is often found at the edges of rhyolytic lava flows considered obsidian. The lack of crystal growth is explained by the inhibition of atomic diffusion through this sticky lava. Obsidian is tough, fragile and amorphous, so it has strong crack corners. The slicing and piercing instruments were previously used and were experimentally used as surgical scalpel blades.
Obsidian is a rock formed by the rapidly cooling lava, the material father. Extensive obsidian formation can occur if felsic magma cools quickly at the corners of a volcanic dome or rift lava stream, or if the lava cools when a sudden touch of water or wind occurs. Obsidian can be intrusive as the felted lava cools at the edges of the deck.
Obsidian consists of about 70% or more silicone (silicon dioxide) that has not crystallized. It is like granite and rhyolite that have also been chemically pre-frozen. Since obsidian is not a mineral crystal, technically it is not a true “stone”. In fact, it is a clotted fluid with small amounts of microscopic and unclean microscopy. Obsidian with a typical hardness of 5 to 5.5 is relatively mild at the level of mineral hardness. In contrast, quartz (crystallized silicon dioxide) has a hardness of 7.0.
Why Obsidian is Black?
Pure obsidian is generally black, although the color differs from the presence of impurities. Jadeite can be light gray to black with iron and other transformation ingredients. Most of the black obsidians are nanoinclusions of magnetite, iron oxide. Very few obsidian specimens are nearly colorless. In some rocks, the incorporation of the mineral cristobalitis into the black glass of tiny yellow radially clustered rocks produces a stained (snowflakes) image.
Obsidian can contain patterns of lava gas bubbles that overlap with the layers formed when the molten rock was cooled. These bubbles can generate thrilling blows like a golden blade (obsidian blade). The inclusion of magnetite nanoparticles that create thin film interference produces an iridescent, rainbow shine. Mexican rainbow colored obsidian contains hedenbergite-oriented nanorods which, by interfering with thin layers, produce a rainbow dispersion effect.
The different colors of obsidian are the result of several factors. There are very few distinct types of obsidian or microscopic mineral crystals. Obsidian red or brown usually produces small crystals or inclusions of hematite or limonite (iron oxide). The tar-black obsidian types likely produce numerous microscopic crystals of minerals such as magnet, hornblende, pyroxene, plagioclase, and biotite in combination with smaller pieces of rock. The distinctive blue, green, purple, or brown hues of rainbow obsidian can be obtained under the microscope of many types of feldspar.
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Recipe for Rotary Tumbling
Snowflake obsidian is extremely “soft” so you do NOT need to use coarse grit.
Medium Grit Step:
Two medium grain (150/220) tablespoons per pound of rock and ceramic material. Add enough water until it barely covers the rocks. You want 50-60% media and 40-50% obsidian. Let it drum for 5-8 days. Rinse, clean and check the material. Dry longer if a smoother, more shaping effect is desired.
Fine grain step:
If more media is needed to maintain the capacity of the barrel, it should be broken in, not new, rough-edged media. Again, two tablespoons of fine grain (500F) per pound of rock and ceramic material. Add water below the top of the rocks. Fall for five days.
IMPORTANT: Make sure the barrel and stock have been thoroughly cleaned. Any litters carried over from the previous step will likely ruin your shine. Don’t add any more media at this point.
Measure two tablespoons of TXP Polish per pound of rock and ceramic material. Fill with water just below the top of the rocks. Fall for seven days.
Polish as needed:
Jasper, agate and petrified wood usually polish well. However, polishing this material after the polishing step can often improve the gloss of the finished stones. If you want to try polishing to see if it improves the look of your polished stones, complete polishing instructions can be found here.
Vibratory Tumbler Recipe:
Coarse grain (60/90 mesh):
Coarse gravel is not used in the vibrating rock tumbler.
medium grain size (150/220 mesh):
Obsidian is a fragile material that breaks easily or splashes. It is recommended to use a minimum of 50% of ceramic substrates. The use of this medium will help the barrel roll over and help deliver sand to all surfaces of the rough material. The more media, the better the cushioning.
After loading the bowl to the manufacturer’s recommended level, add 1 tablespoon of litter for every two pounds of material, including nutrients, in the bowl. While the cup is running, slowly add water until the material has a thin layer of wet sand and the tumbling in the cup is smooth and fast.
Check the bowl every 8-12 hours to make sure operation is still good. If the action has slowed down, add more water until the action is back to normal. If the mud becomes too thick, it will be necessary to rinse the material and pan completely. After rinsing, add fresh cat litter and water and start mixing again. If you are satisfied with the shape and smoothness of the stones, you end up with a medium grain. Usually 2-3 days for us with this material. Thoroughly clean the material and bowl before moving on to a fine grain.
Fine grain (500F mesh):
Place the material back into the cup cup, then add enough polished ceramic material to bring the load to the manufacturer’s recommended operating level. This will bring you closer to the recommended 50% ceramic substrate. Add one tablespoon of porridge for every two pounds of material in the bowl. Then turn on the cup and slowly add water until the material has a thin layer of wet sand and the tumbling in the bowl is smooth and fast.
Open the bowl every 8-12 hours to check if the operation is still good. We usually work on fine sand for 48 hours. Thoroughly clean the bowl and material before proceeding to the polishing stage.
Polish (# 61 Rapid Polish or TXP):
Put the cleaned material back into the bowl. If necessary, add pre-polished ceramic media to bring the bowl to the operating level recommended by the manufacturer. Add one tablespoon of porridge for every two pounds of material in the bowl. Turn on the cup and slowly add water. Stop adding water when the material has a thin coat of wet varnish and the tumbling is smooth and fast.
As with the sandblasting steps above, open the bowl every 8-12 hours to check the tipping action. Add water if it has slowed down. We found that 48 hours is usually the time it takes to get a good shine in
Polish as needed:
The rough one usually needs great polishing. However, polishing this material after the polishing step can often improve the appearance of the finished stones. If you want to try polishing to see if it improves the look of your polished stones, complete polishing instructions can be found here.
Finished Snowflake Obsidian Tumbled Stones:
If you don’t want to turn this snow obsidian yourself, we usually have medium sized pebbles made of this material for sale for pounds. You will find them here.
medium Grit (150/220 mesh): Obsidian is a delicate material that can be easily crushed or chipped. It is recommended to use a minimum of 50% of ceramic substrates. The use of this medium will help the barrel roll over and help deliver sand to all surfaces of the rough material. The more media, the better the cushioning.
How hard is obsidian
Futures speculation: Black imagination and art
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