Gemstone inclusions are a trapped foreign body, mineral, or fluid within a gemstone's structure. These materials get trapped inside the stone during their formation, often only visible with a microscope or 10x loupe, whilst others can be seen with the naked eye.
Inclusions can be anything from other crystals (like a diamond within a diamond) to liquids, gases and even once-living organisms. The term also refers to other features within a gemstone including colour distribution, fractures and cleavages, and structures related to the formation of the material.
Crystals can form either before or at the same time as the host stone but fluid inclusions, excluding air-filled fractures and cleavages, form during growth.
Fluid inclusions can hold gas, or a combination of both, and may even encapsulate crystals. These inclusions are thought to form when stones fracture during their growth. During this process, fluids containing the same chemical components as the host stone are drawn into the cracks, helping to partially repair them.
Petroleum inclusion in quartz with methane gas bubble. Photo credit: Leona Fish.
WHY ARE GEMSTONE INCLUSIONS IMPORTANT?
Inclusions can be great indicators in identifying and differentiating between gemstones. For example, Tiger Stripe Inclusions (see below for full description) in a purple stone can often be an indicator that the stone is Amethyst, however this is not conclusive and must be supported by additional, relevant testing.
Gemstone inclusions can reveal whether the stone is natural or synthetic.
Gemstone inclusions can also provide clues about the gem's geological history, verifying the processes involved during their formation. For example, inclusions in diamonds can provide insights into the Earth’s mantle and supply evidence of the rocks in which they formed and the conditions in which they grew.
Some inclusions only occur in a single gemstone species or even in a single mine!
HOW DO YOU VALUE A GEMSTONE WITH INCLUSIONS?
Inclusions play a crucial role in determining a gemstone’s value. Some inclusions may decrease a gemstone’s value, for example fracture and cleavage inclusions in a diamond will affect the clarity, reducing the value. Other inclusions can increase the value, especially if they contribute to a desirable visual effect or are characteristic of a specific gemstone type.
Below are examples of the various inclusions that can be found in gemstones:
CHATOYANCY (CAT’S EYE EFFECT)
Chatoyancy is a visual effect caused by parallel inclusions, such as fibres, needles, or hollow tubes within the stone. The effect occurs as a band of light appearing to move across the surface of the gemstone whilst the stone is turned. Often seen in Tiger’s eye, chrysoberyl, quartz.
ASTERISM (STAR EFFECT)
Asterism occurs when light reflects off intersecting inclusions, forming a star on cabochons. This effect is often seen in star Sapphires, Rubies, Garnets, Diopside, Spinel and Quartz, and is enhanced by the cabochon cut, which provides a smooth, rounded surface for the star to appear.
RUTILE INCLUSIONS
Some gemstones contain fine needle-like inclusions of other minerals, creating eye-catching patterns. One of the notable examples of Rutile is in gold Rutilated Quartz. Rutile inclusions are also responsible for Chatoyancy and Asterism effects.
HORSETAIL INCLUSIONS
Horsetail inclusions are a desirable feature found in demantoid garnets. These inclusions appear as delicate, feathery, radiating fibres that resemble the shape of a horse’s tail. These inclusions are commonly seen in demantoid garnets from Russia and are a diagnostic feature.
AMBER INCLUSIONS
Amber is formed thousands of years ago from sap periodically seeping out of the damaged part of a tree. They can be any type of organic debris including plants, animals, liquids, inorganic compounds, or various gases. The organic debris gets trapped and preserved within the seeping liquid sap which hardens into Amber. Insect inclusions in particular are highly prised.
TIGER STRIPE INCLUSIONS
Tiger stripe or zebra stripe, is a unique type of inclusion made up of distinctive wavy stripes, often found within Amethyst. The inclusion can exhibit an iridescent quality and may feature three distinct sectors that reflect the mineral's trigonal symmetry.
CENTIPEDE INCLUSIONS
Centipede inclusions can only be found in Moonstones. They are tiny tension cracks within the stone appearing as thin, elongated shapes resembling a centipede's body and legs. Their presence is due to stress within the moonstone's layered structure.
LILYPAD INCLUSIONS
Lily pad inclusions are distinctive, reflective, disk-shaped inclusions found in Peridots. They are essentially stress fractures that form around a small crystal, often chromite, giving them the appearance of a lily pad floating on water. When light strikes these fractures at the right angle, they create a captivating visual effect.
DENDRITIC INCLUSIONS
Dendritic inclusions are distinctive patterns resembling trees or ferns that occur within minerals and rocks. They are created by the branching growth of minerals such as iron or manganese oxides during the crystal formation process. Often found in agate and quartz, enhancing the stones' visual appeal with their unique designs.
HOW TO IDENTIFY GEMSTONE INCLUSIONS
Valuers use a 10x Jewellers Loupe and Microscope to analyse and identify inclusions within gemstones that are not visible to the naked eye.
It is also beneficial to use a variety of light sources during examination including brightfield, darkfield, polarized light, and fibre optic illumination. This lets you see how the gemstone reacts to a controlled pathway of light, revealing valuable information about the inclusions. Each lighting type provides unique insights, highlighting aspects that may be overlooked when using only a single lighting environment.