Ask-A-Geologist #8: What is obsidian rock used for?

Hi everyone! Today I am back with another fabulous question from one of our readers. Today’s question is about obsidian. John asks: What is obsidian rock used for?

Hi John:  In ancient times, obsidian was used to made crude knives and tools. Today, obsidian is used in the production of rock wool, knives and jewelry.

There are two types of obsidian that might interest you. There is the standard, solid black obsidian and then there is snowflake obsidian. Obsidian is an extrusive igneous rock meaning that it forms outside the volcano. When the magma is ejected from the volcano, it cools so quickly that it forms a black glass, which is obsidian. This rock has a conchoidal fracture and is very shiny. Snowflake obsidian is actually the same as black obsidian; however, it has areas that have started to break down. When the mineral breaks down, it forms these white areas that form in the shape of snowflakes, which is why they call this snowflake obsidian. This process is called devitrification and the white spots are actually the mineral cristobalite, which is a form of quartz.

Remember to keep your questions coming in through the Dig into Geology section on our website or email us at Please comment below and subscribe to our channel so that you get the next installment of ask a geologist. Until next time, rock on everybody.


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