Introduction
An inclusion is anything inside a diamond that is not diamond. It might be a crystal of garnet, sealed in place when the diamond grew around it three billion years ago. It might be an internal fracture caused by stress during the stone's residence in the mantle. It might be a wisp of fluid trapped along a growth plane. Whatever its nature, an inclusion is a record — of the conditions, chemistry, and events that shaped the stone.
For the gemologist grading a diamond, inclusions are the primary determinant of clarity — one of the four Cs. For the geologist studying the deep Earth, inclusions are irreplaceable samples of mantle material, sealed in the hardest natural container known and preserved unchanged across geological time. For the buyer, inclusions are the features that make every diamond unique, and the characteristics that most directly affect a stone's transparency, brilliance, and price.
What Inclusions Are
Inclusions in diamonds fall into two broad categories: mineral inclusions (solid crystals) and growth features (internal structures created during the diamond's formation or subsequent history).
Mineral inclusions are fragments of other minerals that were present in the mantle environment when the diamond crystallised. As the diamond grew from carbon-bearing fluids, it engulfed these minerals, trapping them inside an impermeable shell. Once sealed, the included mineral is protected from any chemical or physical change — it remains exactly as it was at the moment of encapsulation.
Growth features include internal graining (visible lines created by irregular crystal growth), twinning wisps (disturbances along twin planes in macle crystals), feathers (internal fractures, often following cleavage planes), and clouds (clusters of microscopic pinpoint inclusions too small to resolve individually). These are not foreign minerals but structural characteristics of the diamond itself.
Both categories affect clarity grading, but mineral inclusions are the ones that carry geological information. A feather tells you the stone was stressed. A crystal of pyrope garnet tells you the temperature, pressure, and chemistry of the mantle where the diamond formed.
The Mineral Evidence: What's Inside
The specific minerals found as inclusions in diamond are diagnostic of the host rock where the diamond formed. Two suites dominate.
Peridotitic inclusions come from diamonds that formed in peridotite, the dominant rock of the upper mantle. The characteristic minerals are:
- Olivine (forsterite) — the most common peridotitic inclusion, a green magnesium silicate
- Pyrope garnet — a magnesium-rich garnet, often with distinctive purple-red colour, sometimes chrome-rich (chrome pyrope)
- Chrome-spinel — a dark, opaque chromium-bearing oxide
- Enstatite — an orthopyroxene, less common but diagnostically significant
- Chrome diopside — a vivid green clinopyroxene
Eclogitic inclusions come from diamonds that formed in eclogite — metamorphosed oceanic crust that was subducted into the mantle. The diagnostic minerals are:
- Orange garnet (almandine-pyrope) — compositionally distinct from peridotitic garnets
- Omphacite — a sodium-rich clinopyroxene characteristic of eclogite
- Coesite — a high-pressure polymorph of quartz, found only in diamonds from specific depth ranges
- Rutile — titanium oxide, occasionally present
The mineral suite immediately tells a geologist whether the diamond formed in ancient mantle root rock or in recycled oceanic crust — two fundamentally different geological environments. This distinction is invisible in a polished stone under normal examination, but it is written in the inclusions.
Inclusions as Time Capsules
The scientific value of diamond inclusions is extraordinary. Because diamond is chemically inert, impermeable, and the hardest natural material, it preserves its inclusions in a state of perfect isolation. A garnet crystal trapped inside a diamond 3 billion years ago has been protected from every geological process that has occurred since — metamorphism, weathering, plate tectonics, surface chemistry. It is a pristine sample of the deep mantle, delivered to the surface in its original state.
Researchers use radiogenic isotope dating of these inclusions — primarily samarium-neodymium and rhenium-osmium systems — to determine when the diamond formed. The inclusion's isotopic ratios record the moment it was sealed inside the diamond, providing a formation age for the host stone.
These ages reveal that diamond formation is episodic. A single kimberlite pipe may contain diamonds from multiple formation events, spanning billions of years. The Kaapvaal craton in southern Africa, for example, hosts diamonds with ages ranging from 3.3 billion to roughly 90 million years — recording intermittent diamond-forming episodes across nearly the entire history of the Earth.
Inclusion chemistry also reveals the temperature and pressure of formation. Mineral geothermobarometry — calibrated relationships between the chemical composition of coexisting minerals and the conditions under which they equilibrated — allows scientists to estimate the depth and temperature at which a specific diamond grew. A diamond containing both garnet and orthopyroxene, for instance, permits calculations that can pinpoint its formation depth to within 10 to 20 kilometres.
Inclusions and Clarity Grading
For the diamond buyer, inclusions matter because they determine clarity — and clarity directly affects transparency, light performance, and price.
The GIA clarity scale evaluates inclusions based on five criteria: size, number, position, nature, and relief (how visible the inclusion is relative to the surrounding diamond). The scale runs from Flawless (FL) to Included (I3):
- Flawless (FL) and Internally Flawless (IF): No inclusions visible at 10x magnification
- Very, Very Slightly Included (VVS1, VVS2): Inclusions extremely difficult to see at 10x
- Very Slightly Included (VS1, VS2): Inclusions minor, ranging from difficult to somewhat easy to see at 10x
- Slightly Included (SI1, SI2): Inclusions noticeable at 10x
- Included (I1, I2, I3): Inclusions obvious at 10x and may affect transparency and brilliance
Position matters as much as size. A small inclusion directly under the table facet — the large flat facet on top — is more visible and more damaging to light performance than a larger inclusion near the girdle, where it may be hidden by a prong setting. Experienced buyers and jewellers evaluate "eye-cleanliness" — whether inclusions are visible without magnification — as a practical threshold distinct from the laboratory grade.
The type of inclusion also affects appearance. A dark crystal of chrome-spinel stands out more than a transparent crystal of olivine. A feather that reflects light internally can create a visible flash. A cloud of pinpoints may be individually invisible but collectively reduce transparency.
The Inclusion Plot: A Diamond's Identity
Every GIA grading report for a diamond of 1.00 carat or larger includes a clarity diagram — a schematic drawing of the diamond showing the position, size, and type of each significant inclusion. This plot serves as an identification tool: because no two diamonds share the same inclusion pattern, the clarity diagram functions as a fingerprint.
The plot uses standardised symbols: red for internal characteristics (inclusions), green for external features (blemishes). Crystal inclusions are marked with small circles or shapes, feathers with curved lines, clouds with dotted outlines. By comparing a diamond against its grading report diagram, a jeweller or appraiser can confirm the stone's identity — useful for insurance, resale, and verification after setting.
This uniqueness is not a grading abstraction. It is a physical fact. The precise arrangement of mineral crystals, growth features, and internal fractures in any given diamond is unrepeatable — a geological fingerprint formed over billions of years.
Living With Inclusions: A Buyer's Perspective
Inclusions are sometimes described as "flaws," but the term is misleading. An inclusion is a natural feature of a natural stone — evidence of its geological origin. A diamond without any inclusions (Flawless grade) is genuinely rare, and priced accordingly. But the practical difference between Flawless and VS2 is invisible to the unaided eye in a well-cut stone.
The informed buyer evaluates inclusions not as defects but as variables to be managed. Key considerations include:
- Eye-clean SI1 stones often represent the strongest value in the market: graded below the premium VS tier but visually identical to higher grades when viewed without magnification.
- Inclusion position determines visibility. A stone graded SI1 with inclusions concentrated under the table is a different proposition from an SI1 with inclusions near the pavilion edge.
- Dark inclusions are more visible than white or transparent ones at any given clarity grade.
- Feathers reaching the girdle can present durability concerns if they are large enough to act as stress concentrators during setting or wear — though this is uncommon in stones that have survived geological transport.
Every diamond tells a story in its inclusions. The buyer who understands that story makes a more informed choice — not just about clarity grades and price brackets, but about the geological reality of the stone in hand.
Summary
Inclusions are the internal record of a diamond's formation — mineral crystals and growth features that document the conditions, chemistry, and age of the deep mantle environment where the stone crystallised. For science, they are irreplaceable samples of Earth's interior. For gemology, they define the clarity grade. For the buyer, they make every diamond unique. Understanding inclusions transforms them from perceived imperfections into what they actually are: nature's fingerprint, sealed in the hardest natural material on Earth.