Geological origin of gemstones

Gemstones are formed below the Earth's surface and can sometimes show traces of other minerals, called inclusions.

Gemstones are a product of the earth. Some, like diamond and zircon, were formed deep in the earth and brought to the surface by explosions of molten rock. Many, like topaz, tourmaline and aquamarine, crystallised slowly from hot fluids and gases as they cooled and solidified, far below the surface of the earth. Others formed from liquids filtered into cracks and pockets in rock, like Australian opal. Some, like garnet and jade, formed when rocks were heated and pressurised by earth movements, and recombined to form new, different minerals.

Most minerals contain visible traces of their genesis - perhaps tiny crystals of other minerals that were caught up in the growth of the larger host crystal, or formed simultaneously as it grew, internal fractures that have been partially healed during growth, or traces of earlier growth stages, marked by zoning. Gemmologists use the word inclusions to describe these and other internal phenomena.

Inclusions in a gemstone, viewed through a microscope or a 10x loupe, can give information about the geological environment in which the mineral was formed.

Inclusions can sometimes identify gemstones and they can prove whether the stone is natural or synthetic. Inclusions can also tell us where a gemstone comes from. For example, the hairs in an amber bead blew into the sticky resin from an oak tree flower in Spring or early Summer, in the Baltic Sea region, 20 - 40 million years ago.

Some gemstones rely on their inclusions for their effect, like aventurine quartz, or star sapphires. Green aventurine quartz is actually colourless quartzite, filled with tiny flakes of green fuchsite mica that lend it colour.

Star sapphires are filled with elongated crystals of rutile (often called 'silk') arranged in three directions parallel to the faces of the hexagonal prism. When such a stone is cut in a dome shape, with its base parallel to the basal plane of the crystal, a six-pointed star (or sometimes a twelve-pointed one) is seen in the reflected light of the sun or a spotlight.