Obsidian is dense volcanic glass, usually rhyolite in composition and typically black in color. Compared with window glass, obsidian is rich in iron and magnesium; tiny (<.005 mm) crystals of iron oxide within the glass cause its dark color. 
Obsidian is often formed in rhyolite lava flows where the lava cools so fast that crystals do not have time to grow. Glass, unlike crystals, has no regular structure and therefore fractures in smooth conchoidal (curved) shapes. The intersections of these fractures can form edges sharper than the finest steel blades. For this reason, obsidian was used by many native cultures to make arrowheads and blades.

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PHYSICAL CHARACTERISTICS:
Color is dark green to dark brown and black, also can show sheens of gold or green, yellow, blue and/or purple coloration. Sometimes with white inclusions (Snowflake Obsidian).
Luster is vitreous.
Transparency: Obsidian is translucent in any stone of appreciable size.
Crystal System does not apply because obsidian is amorphous.
Habits include compact nodules or as massive layers between other volcanic rocks
Fracture is conchoidal.
Hardness is 5 - 5.5 (much softer than quartz).
Specific Gravity is approximately 2.6 (average)
Streak is white.
Other Characteristics: Generally lacks open voids or large bubbles like other volcanic rocks.
Notable Occurrences include Italy; Mexico; Scotland; Arizona, Colorado, Texas, Utah and Idaho, USA, as well as the Cascade Volcanic Mountain range and its associated lava beds, which stretches from Northern California into Washington state.
Best Field Indicators are color, fracture, flow bubbles, softness, association with other volcanic rocks and lack of crystal faces.

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More about color of obsidian
The colors in obsidian result from the oxidation state of the chemical elements within the tiny minerals that are finely dispersed in the glass. Black color results chiefly from magnetite, Fe304. If the obsidian is highly oxidized, then the glass may contain hematite, which provides a reddish hue. Variations in the oxidation state of the iron (Fe) varieties imparts a slight greenish hue. Some obsidian is banded, a consequence of oxidation on a flow surface being folded into the lava as it continues to move.

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