OBSIDIAN

An extrusive rock with a glossy appearance. It is principally composed of quartz and feldspar and is a black/grey colour

It's glassy appearance is due to it's method of formation. If lava contains 1% water it cools more quickly than usual, in fact too quickly for crystallization. It is very silica rich and similar in composition to granite and rhyolite.

It can be found in Cumbria (UK) and is also abundant in Yellowstone Park (USA). In ancient times it was used as a tool due to it's sharpness.

GYPSUM

A member of the sulphate group with the formula CaSo4H2O. It is very soft (2 on the mohs hardness scale) and is usually white. It's name is derived from the word gypus which is the Greek for chalk.

Fine grained gypsum is known as alabaster and this is widely used in industry particularly as a building material.

It forms by evaporation of salt water in lakes.