Solid Inclusions
Crystal inclusions - identifying mineral guests trapped within gemstones during formation.
By gemmology.dev editors
Last updated
microscopy identification crystals origin
Introduction
Solid inclusions are crystals of other minerals trapped during the host gem's growth.
The textbook classifies inclusions into three temporal categories:
- Protogenetic: Formed before the host crystal grew. These are pre-existing mineral
particles that became engulfed during growth (e.g. pyrite in emerald). - Syngenetic: Formed at the same time as the host material. Most liquid and gaseous
inclusions are syngenetic, but solids can also be syngenetic (e.g. calcite in emerald).
These give information about the growing environment of the host. - Epigenetic: Formed after the host crystal stopped growing. These include minerals
that fill fractures, stress cracks caused by heat treatment, and alteration products
of existing inclusions (e.g. fractures around zircon crystals in heat-treated sapphire).
These mineral "guests" provide crucial evidence for identification and
origin determination.
Common Solid Inclusions by Host Gem
Diagnostic Crystal Inclusions
| Host Gem | Common Solid Inclusions | Significance |
|---|---|---|
| Ruby/Sapphire | Rutile silk, zircon halos, calcite, apatite | Origin indicators |
| Emerald | Pyrite, calcite, tremolite, mica, chromite | Origin-diagnostic |
| Diamond | Garnet, olivine, chromite, graphite | Mantle origin proof |
| Peridot | Chromite (black crystals), ludwigite needles | Species-diagnostic |
| Garnet | Rutile, apatite, rounded crystals | Varies by species |
| Spinel | Octahedral crystals, zircon, calcite, apatite | Natural indicator |
| Tourmaline | Trichites, growth tubes | Common features |
Corundum (Ruby & Sapphire)
Rutile Inclusions
Rutile (TiO2) is the most common solid inclusion in corundum:
- Short silk - Fine, short needles (Burmese origin indicator)
- Long silk - Longer needles at 60° angles (Sri Lankan indicator)
- Dense networks - Heavy silk coverage (Thai/Cambodian stones)
- Dissolved silk - Dotted remnants indicate heat treatment
Other Crystal Inclusions
| Inclusion | Appearance | Origin Significance |
|---|---|---|
| Zircon with halos | Round crystals with stress fractures | Common, suggests high-temp formation |
| Calcite | Rhombohedral crystals | Burmese ruby indicator |
| Apatite | Hexagonal prisms | Common in many origins |
| Boehmite needles | Fine, oriented needles | Heat treatment indicator |
| Spinel | Octahedral crystals | Occasionally seen |
Emerald Crystal Inclusions
Emerald inclusions vary significantly by origin and provide key diagnostic evidence.
Emerald Solid Inclusions by Origin
| Origin | Characteristic Crystals |
|---|---|
| Colombia | Calcite rhombs, pyrite cubes, albite |
| Zambia | Black mica flakes, amphibole needles, blocky crystals |
| Brazil | Biotite mica, pyrite, chromite |
| Afghanistan | Mica books, actinolite needles |
| Russia (Urals) | Actinolite, mica, chlorite |
Diamond Crystal Inclusions
Diamond inclusions provide information about formation conditions deep in the
Earth's mantle. They are syngenetic, crystallizing alongside the diamond.
| Inclusion | Appearance | Significance |
|---|---|---|
| Garnet (pyrope) | Red or orange crystals | Peridotitic environment |
| Olivine | Green crystals | Common mantle mineral |
| Chrome diopside | Green crystals | Eclogitic environment |
| Chromite | Black, opaque | Very common |
| Graphite | Black, metallic | Same carbon, different structure |
| Diamond-in-diamond | Colourless crystal | Growth interruption |
Garnet Family Inclusions
Demantoid Garnet
Demantoid (andradite) from Russia shows the diagnostic "horsetail" inclusion:
- Horsetail: Radiating fibres of byssolite (actinolite)
- Fibres curve outward from central point
- Diagnostic for Russian (Ural) demantoid
- Namibian demantoid may lack horsetails
Tsavorite Garnet
Tsavorite (green grossular) from East Africa shows:
- Graphite plates: Flat, dark inclusions
- Actinolite needles: Elongated crystals
- Growth tubes: Hollow channels
- Generally cleaner than hessonite
Other Garnets
| Species | Common Crystal Inclusions |
|---|---|
| Rhodolite | Rutile needles, apatite crystals, zircon |
| Almandine | Rutile silk, quartz, apatite |
| Pyrope | Often eye-clean; chrome diopside, olivine |
| Spessartine | Apatite, quartz crystals |
Spinel Crystal Inclusions
| Inclusion | Description |
|---|---|
| Octahedral crystals | Small spinel crystals within host |
| Zircon with halos | Radiation damage halos around zircon |
| Calcite | Rhombohedral crystals |
| Apatite | Hexagonal prisms |
| Graphite | Flat, dark plates |