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What are Inclusions and Impurities? (geology)

fine golden inclusions within clear quartz

Inclusions in Rocks, Minerals and Gemstones

In geology an inclusion is something that becomes trapped during the early stages of a rock or mineral's formation.  It's often large enough to be seen with the naked eye.

In a gemstone an inclusion is a characteristic that's either enclosed within the stone or that reaches the surface from within.

Inclusions in rocks, minerals and gemstones can be almost anything.  The most common inclusions are water, gas, petroleum or other minerals.

In the case of amber, inclusions are usually insects or plant matter.  With that said, amber is neither a rock or mineral because it's fossilised tree resin.inclusions of black tourmaline in a colourless transparent stoneInclusions are one of the most important factors when determining a gemstone's value.  Should the inclusion affect clarity it will reduce the value of the stone.

In gemstones such as star rubies, star sapphires and varieties of included quartz, inclusions are the reason for the increase in value.

What are Impurities?

Inclusions are not the same as impurities.  An impurity is part of the crystal structure of a rock, mineral or gemstone.  It cannot usually be seen.

The iron in clear quartz that turns it into amethyst is an impurity.  The presence of the mineral rutile or tourmaline in quartz is an inclusion.

The red spots in bloodstone are impurities of hematite.  The green in peridot comes from impurities of iron.  The golden spots in lapis lazuli are inclusions of pyrite.

The pinkish red colour of rhodochrosite comes from impurities of manganese.

In its purest form quartz is colourless.  The presence of various impurities produces amethyst, rose quartz, citrine and smoky quartz.

The mineral corundum turns red with impurities of chromium.  Replace chromium with iron and it turns yellow.  With titanium and vanadium it turns blue.  Red corundum is ruby all other colours are sapphire.

The colour of malachite comes from impurities of copper.  The varying shades of blue in turquoise come from impurities of copper and aluminium.

In its purest form the mineral beryl is colourless.  Various impurities produce shades of red, green, yellow and blue.  Green beryl is emerald, blue is aquamarine.

Other factors that affect colour in rocks, minerals and gemstones include heat, the orientation of the crystals and the presence of a structural imperfection.  That may be damage or an anomaly that has occurred during its formation.

When inclusions are present in abundance they can change a stone's colour completely.  This can be seen in the mineral aventurine and also in rutilated quartz.

When rutilated quartz is heavily included with golden crystals of rutile the stone can look distinctively golden.  Were the stone to be inclusion free the quartz would be clear and colourless. 

Another example is the orange-brown variety of goldstone.  This man-made stone is produced from glass filled with crystallised copper.  The glass is colourless but the copper inclusions turn the stone orange-brown.

Article Pictures

The first picture in our article is quartz with inclusions of rutile and clay.  The photo is clickable and redirects to the original image.  Courtesy of Steve (singingstone48).

The second picture is included quartz (quartz with inclusions of black tourmaline).

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