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Background: Kajjali, the black powder produced by triturating purified mercury with purified sulphur, is the starting point for most mercurial formulations of Rasa Shastra, including kupipakva rasayanas such as Rasasindura and Makaradhwaja and many compound rasa preparations. Its quality determines the safety of everything made from it. Objective: To review the chemistry of kajjali formation, the analytical evidence on its composition and particle size, and the evidence on its toxicity. Methods: A critical narrative review of characterisation studies using X-ray diffraction, electron microscopy, elemental and thermal analysis, and of in vitro, zebrafish and rodent studies on kajjali and kajjali-derived products. Results: Trituration converts metallic mercury and sulphur into black cubic mercuric sulphide (metacinnabar, β-HgS) mixed with unreacted orthorhombic sulphur. With adequate trituration, free mercury falls to undetectable or minimal levels. Reported particle sizes are in the range of tens to a few hundred nanometres for well-triturated samples; in one study, particles narrowed from 185–661 nm at 22 hours to 100–171 nm at 120 hours. The sulphur ratio governs the proportion of unreacted sulphur (TGA mass loss 45.9% for 1:1 and 80.5% for 1:6) and crystallite size. HgS is far less bioavailable and toxic than mercuric chloride or methylmercury, and zebrafish and cell studies suggest lower toxicity of processed mercurials, but long-term mammalian data are lacking. Conclusion: Kajjali is a chemically definable, testable intermediate. Instrumental end-points for free mercury, phase composition and particle size should replace reliance on organoleptic tests alone.