![]() Though felsic lava is mostly silicates, it contains some iron minerals like magnetite and hematite. Mahogany obsidian has curving ribbons of brownish color running through the darker gray-black obsidian. Lapidary artists use snowflake obsidian to create domed cabochons, ring stones and pendant stones. Time and a slower rate of lava cooling, allow some crystal molecules of silica to draw together to form micro-crystal clusters of the mineral white cristobalite, a form of quartz. This dark obsidian has what appears to be very small flower-like spherules of minerals. One of the more popular types of obsidian is called snowflake obsidian. These tiny spherules of crystals, depending on size, create a variety of useful obsidian. Obsidian is largely amorphous or non-crystalline but it will often develop tiny sub-microscopic spherules or mineral grains. By the nature of the material, lava tends to be fine-grained which contributes to its dark color but inhibits crystal growth. Obsidian usually appears black or gray but it is not always glass clear. Today we call this knapping and at some shows, artists demonstrate the art of knapping. Broken obsidian will have sharp edges so it did not take long for humans to figure out they could strike the edges of an obsidian piece to make their tools.Įventually, people learned that applying steady pressure along the edge of obsidian would achieve the shape and sharpness they wanted to produce. This was obviously of great use to early hunters who could make sharp weapons or tools including knives. Like glass, when obsidian breaks, the pattern that forms is a curving shell-like surface that is razor-sharp. It was undoubtedly the conchoidal fracturing of obsidian that brought it to the attention of early humans. It is brittle but can be lustrous and beautiful. It comes in several different forms that are attractive and of interest to lapidary artists who enjoy the challenge of working with a substance that has a high conchoidal fracture property. Anyone who has seen objects made of obsidian knows it is always glass-like, dark and clear. So, true obsidian is not a mineral but a glass. When it cools rapidly in contact with water, and even air, the mineral crystals usually cannot form. The composition of felsic lava is primarily silica. A prime example of this is Obsidian Cliffs in Yellowstone National Park, a flow that was exposed by road building and is many feet thick of solid obsidian. Mafic lava can form what is called basalt glass which is dark and forms as a lining of vesicles or gas pockets and around the edges of flows where air can cool the material more rapidly.īasalt glass tends to be relatively small whereas obsidian formed from felsic lava can occur in huge layers. ![]() These two basic types of lava can produce what we call obsidian, but true obsidian forms from felsic lava cooled rapidly. You see this in the lava flows of Hawaii that flow rapidly spreading over the land often solidifying on the surface and continuing to flow underneath the new crust. Mafic lava is more fluid and, when hot enough, flows almost like a river. This inhibits atomic diffusion, the movement of molecules needed to form crystals. Lava that’s high in silica tends to be viscous, move slowly and develop thicker, solid flows. Rhyolite rock is a prime example of high silica lava. ![]() They are composed of quartz and feldspar so they are lighter hued. Basalt is a common form of mafic lava.įelsic lavas are just the opposite, high in silica and low in iron and magnesium minerals. Mafic lavas are dark, dense and made primarily of dark iron and magnesium minerals but are low in the lighter elements that make up the silica minerals and rocks. There are two major types of lava based on their mineral content and chemistry - mafic and felsic. Under other conditions, it can be vitreous, glassy and gemmy. Under the right conditions, we can find gem agate, fire agate or even mineral crystals. Depending on its gas content, the hardened lava can be vuggy meaning it contains pores or voids. Its layers are dense, seldom revealing any crystals. Normally when lava flows, it ends up as thick layers of solid rock. Is it a mineral? The answer lies in how it’s formed. Obsidian is a volcanic glass that’s formed when molten lava cools suddenly.
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