Obsidian, large size specimen

Description

Large Obsidian specimen for the classroom from the Tarawera volcanic eruption of 1886 in the Bay of Plenty Region, New Zealand.

Formed during the eruption of Mt Tarawera near Rotorua in June 1886 at the margins of rhyolitic flows.  Contains pumice and perlite inclusions formed by contact with groundwater.

In the classroom explore differences in structure compared to other rocks from felsic lava such as pumice and rhyolite.

Colour: black to black/green

Fracture: conchoidal

Mohs hardness :5-6

Specific gravity: ~2.4

Specimens are usually no smaller than  110mm in largest dimension and weigh greater than 365g