Inclusions Overview

An inclusion is any internal feature (solid, liquid, gaseous, or structural) formed or trapped within a gemstone during or after its growth. Gübelin and Koivula described inclusions as a gem's "fingerprint" ^[1]: no two stones carry exactly the same internal record, and that record encodes species identity, geographic origin, and treatment history that no surface measurement can provide. The three principal classes are solid inclusions (crystals of other minerals), fluid inclusions (trapped liquids, gases, or multi-phase cavities), and growth features (zoning, twinning, and structural patterns intrinsic to crystallisation). Timing also matters: protogenetic inclusions pre-date the host crystal; syngenetic features formed simultaneously; epigenetic features developed afterwards, such as the discoid stress fractures that radiate from zircon crystals in heat-treated corundum, created when thermal expansion of the zircon exceeded the surrounding sapphire's tolerance above 1600 °C. Reading inclusions at 40–60× under darkfield illumination is the first and most informative step in any identification workflow ^[2].

Inclusions can be classified by their nature and timing of formation ^[2]:

By Nature:

• Solid inclusions - Crystals of other minerals
• Fluid inclusions - Trapped liquids, gases, or combinations
• Growth features - Patterns from crystallization process

By Timing (Relative to Host Crystal):

• Protogenetic - Formed before the host crystal
• Syngenetic - Formed at the same time as the host
• Epigenetic - Formed after the host crystal

Different lighting techniques reveal different inclusion types:

• Darkfield - Best for seeing inclusions against dark background
• Brightfield - Good for colour zoning and growth patterns
• Fibre-optic - Pinpoint light for specific features
• Immersion - Reduces surface reflections, reveals internal features
• Polarized light - Shows strain patterns and twinning