What are Inclusions and Impurities? (Geology)

Inclusions in Rocks, Minerals, and Gemstones

In geology, an inclusion is material that becomes trapped during the formation of a rock or mineral.

Some inclusions are large enough to be seen with the naked eye, while others are only visible with magnification.

Inclusions can be almost anything, with some of the most common being water, gas, petroleum, or other minerals.

Inclusions in amber are usually insects or plant matter. However, amber is neither a rock nor a mineral because it's fossilised tree resin.

The red spots in bloodstone are inclusions of hematite. Quartz often features inclusions of the minerals rutile or tourmaline.

The golden spots in lapis lazuli are inclusions of pyrite.

Inclusions are among the most important factors when determining a gemstone's value. If an inclusion affects clarity, it may reduce the stone's value.

In gemstones such as star rubies, star sapphires, and varieties of included quartz, the inclusions can enhance the stone's value.

What are Impurities?

An impurity is a chemical element present within the crystal structure of a rock, mineral, or gemstone. Unlike an inclusion, it cannot usually be seen.

The iron present in quartz that causes it to become amethyst is an impurity, activated by natural radiation.  

The green of peridot comes from iron impurities. The pinkish-red colour of rhodochrosite comes from manganese.

In its purest form, quartz is colourless, but the presence of impurities transforms it into amethyst, rose quartz, citrine, smoky quartz, and many other quartz varieties.

Impurities of chromium turn corundum red. If chromium is replaced by iron, it turns yellow. With titanium and vanadium, it turns blue. Red corundum is ruby, while all other colours are sapphire.

Malachite's green colour comes from impurities of copper. The varying shades of blue in turquoise are due to impurities of copper and aluminium.

In its purest form, the mineral beryl is colourless, but 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 occurred during its formation.

When inclusions are present in abundance, they can change the colour of a stone completely. This can be seen in the mineral aventurine and in rutilated quartz. Without the golden-coloured rutile crystals, rutilated quartz would not be as yellow.

Another example of inclusions changing the colour of a stone is the orange-brown variety of goldstone. This man-made material is produced from glass filled with crystallised copper. Colourless glass is used, but the abundance of copper inclusions influences the stone's colour.

Article Pictures

The first picture in our article is quartz with inclusions of rutile and clay. Courtesy of Steve (singingstone48 - Flickr).

The second is quartz with black tourmaline inclusions.

Pop-up photos
Amethyst crystals, aquamarine, gas inclusions in quartz: Courtesy of Stan Celestian. The original image of the gas inclusions is here. However, you may have to join Flickr to be able to see it. It's free to join, and there are no additional fees.

Bloodstone: Courtesy of James St. John. Lapis Lazuli: Stone Mania.