Inclusions are internal features formed during or after a gem's crystallization. They include mineral crystals, fluid-filled cavities, growth zones, and healed fractures. Three-phase inclusions (solid crystal, liquid, and gas bubble) are characteristic of Colombian emeralds formed in hydrothermal veins.
Rutile "silk" needles in corundum indicate natural origin and can produce asterism (star effect) when aligned. Gemologists study inclusions at 10x–100x magnification to determine geographic source, formation environment, and treatment history. Inclusions are not merely flaws; they provide objective evidence of authenticity. Heavily included stones may be clarity-enhanced, while synthetics often show gas bubbles or curved growth lines absent in natural material.