Obsidian
Description
Obsidian is a naturally occurring volcanic glass produced when volcanic lava cools rapidly without crystal growth. Due to this lack of crystal structure obsidian can form with microscopically thin edges – hence its ancient use as arrowheads and blades. Pure obsidian, as seen here, is usually dark in appearance, although the colour can vary due to impurities – the dark brown to black of this example is probably due to manganese and iron. The pale, swirling patterns which capture so beautifully its fluid origins are formed by gas bubbles remaining from the lava flow, aligned along layers created as the molten rock was flowing before being cooled.