Inclusion in Corundum may be solid, liquid or gaseous. RWH has made a usefullist of the major inclusions in corundum, reproduced, with some alterations, below. Further details will be found in the discussions of particular localities.
1. Straight angular growth lines following various crystal faces, often in a hexagonal pattern and often featuring associated minute exsolved needles or particles following these growth lines. The lines vary in thickness and spacing, and are never curved (if examined parallel to the face along which they grew), and always lie inside the stone. They are associated with crystal faces, not with polished facets. Sharp lines are seen best with dark-field illumination, or better, immersion with light-field shadowing illumination. Broad bands or ill-defined patches are best seen with immersion and diffused light-field illumination. In rubies, the colour often occurs in treacle-like swirls when looking in directions other than along the crystal faces.
2. Exsolved rutile needles and or hematite plates (silk) forming parallel to the hexagonal prism (three directions, intersecting at 60/120° in the basal plane) often forming dense clouds. The rutile occurs as intergrown twins with re-entrant angles at the broad end while hematite tends to form plates. Sizes vary greatly, some being much longer than
others, some appearing as mere dots, some broad, some narrow. Overhead fibre-optic illumination is often best, looking down the c-axis. Minute exsolved particles are often best seen with the fibreoptic light guide from below or to the side of the stone.
3. Crystals of different minerals of various types, including spinel, apatite, zircon, calcite, dolomite, uranpyrochlore [this name valid in Dana 8 though not in Glossary of Mineral Species 2004], mica group minerals plagioclase, pyrrhotite and other species, best viewed in dark-field illumination or via fibre-optic lighting.
4. Secondary liquid inclusions in patterns of infinite variety and thickness; often referred to as fingerprints or feathers. They are created when fractures are healed by post-formation geological activity. Their patterns may often be wispy or veil-like, and so are easily confused with flux inclusions in synthetic corundum. Their surfaces should be
examined under high magnification with fibre-optic lighting to see if liquid (natural) or flux (synthetic) fills the small channels. As natural stones healed over a much longer period of time, their healing patterns are often far more detailed. The higher viscosity of a flux also produces a coarser and less detailed healing in flux-growns ynthetics.
5. Polysynthetic twinning along the rhombohedron (in three directions, but only two in any one plane) meeting at 86.1 and 93.9. These lie about 30–60 off the c-axis. Growth twins may also be seen along other faces. Immersion and examination between crossed polars will separate true twinning from sharp colour zoning. True twinning planes will show interference fringes and appear light against a dark background.
6. Long white exsolved boehmite needles which form at the junctions of intersecting rhombohedral twinning planes. Thus their directions and angles are the same as that described in 5. Rhombohedral twinning with the boehmite needles has yet to be seen in the flux-grown synthetic corundum and so is extremely important for identification.
8. Wavy parallel cracks (‘fire marks’) near the facet junctions due to overly rapid polishing. These are more commonly seen on synthetic stones, as less care is taken in the polishing process, but may sometimes be seen in natural corundum, too.