Helium.com | Ecology & Environment | Why obsidian and igneous rocks appear smooth and glassy http://www.helium.com/knowledge/291021-why-obsidian-and-igneous-rocks-appear-smooth-and-glassy.xml Helium.com > Ecology & Environment > Why obsidian and igneous rocks appear smooth and glassy en-us Copyright Helium.com Mon, 02 Nov 2009 20:05:49 GMT Mon, 02 Nov 2009 20:05:49 GMT Igneous rocks, one of the three main classifications of rocks, are formed when molten material cools, or in general, when a rock is formed under ex... http://www.helium.com/items/1662438-obsidian-rocks-and-igneous-rocks-smooth-and-glassy Wed, 25 Nov 2009 01:29:00