Friday, December 5, 2014

Mexican Obsidian ... it makes the sharpest of knives!

The Pacific "Rim of Fire" is a region of volcanoes that are periodically erupting lavas and spewing ash into the atmosphere. When these lava flows cool quickly they freeze into a volcanic glass that is called obsidian. The lava may flow into the sea, or a lake, and thus cool quickly. Obsidians tend to be fairly high silica (70% +)of rhyolitic composition. Volcanic regions having occurrences of obsidian have been sought after by ancient man and used to fabricate arrowheads and knives. Obsidian makes the sharpest of knives and even today may be used by surgeons.

Snowflake obsidian depicted here is a favorite lapidary material for making pendants and bead jewelry. The lava has not cooled quickly enough to form all glass, but has allowed white cristobalite spherulites, a variety of silica, to nucleate and grow, and now appear within a glassy matrix.

A fortune teller's crystal ball?? No,a ball of silver sheen obsidian which is probably as good for telling fortunes. The sheen is said to be caused by aligned microscopic gas bubbles. The iridescent rainbow sheen obsidian opposite has been shown to be caused by inclusions of magnetite nanoparticles.

Mexico City is a good place to see and buy a good range of obsidian. The nearby ancient city of the Aztecs, Tiotihuacan, famous for its pyramids of the Sun and Moon, was once the center for obsidian workshops with dozens of shops producing obsidian knives, arrowheads, swords and ornaments. Now-a-days on tour you can visit some local museums or buy replica artifacts and obsidian specimens from the many stall holders.

The most famous location of old Aztec obsidian quarries is at Sierra de la Navajas (Hill of Knives) near Pachuca, not far away.

Another popular form of obsidian is that known as Apache Tears, which is found plentifully in Arizona. They are residual nodules of obsidian found within perlite rock which forms on weathering of the obsidian. These transparent tear-drop forms of obsidian are tumbled polished to make attractive pendants and other jewelry.