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I should have added obsidian to my list for
comparisons:
- 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.
- Best Field Indicators are color, fracture, flow bubbles, softness,
association with other volcanic rocks and lack of crystal faces.
Mark
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