Obsidian is a form of natural glass which forms when volcanic lava cools quickly.
From the independent.co.uk
The natural glass minnow is working good.
From the inrich.com
Less pure silica forms the natural glass obsidian.
From the en.wikipedia.org
These rocks are fine-grained and sometimes cool so rapidly that no crystals can form and result in a natural glass, such as obsidian.
From the en.wikipedia.org
The company offers high altitude, alkaline water in an all natural glass container and is designated as a premium brand by the Georgia Department of Agriculture.
From the sacbee.com
Perlite, a natural glass of volcanic origin that expands when quickly heated, has become increasingly expensive due to costs of mining, transportation, and production.
From the sciencedaily.com
Obsidian, natural volcanic glass, shatters easily and was rarely used.
From the en.wikipedia.org
I fill a large glass hurricane vase with natural items that symbolize the season.
From the sacbee.com
Much of the structure is glass to give plenty of natural lighting during the day.