Obsidian is a naturally occurring volcanic glass formed as an extrusive igneous rock.
It is produced when felsic lava extruded from a volcano cools rapidly with minimum crystal growth. Obsidian is commonly found within the margins of rhyolitic lava flows known as obsidian flows, where the chemical composition (high silica content) induces a high viscosity and polymerization degree of the lava. The inhibition of atomic diffusion through this highly viscous and polymerized lava explains the lack of crystal growth. Obsidian is hard and brittle; it therefore fractures with very sharp edges, which had been used in the past in cutting and piercing tools, and are still used as surgical scalpel blades.
The Trobriand Islands (today officially known as the Kiriwina Islands) are a 450 km² archipelago of coral atolls off the eastern coast of New Guinea. They are situated in Milne Bay Province in Papua New Guinea. Most of the population of 12,000 indigenous inhabitants live on the main island of Kiriwina, which is also the location of the government station, Losuia. Other major islands in the group are Kaileuna, Vakuta and Kitava. The group is considered to be an important tropical rainforest ecoregion in need of conservation.