Explanation

Magnesium burns in air with an intense, bright white light (forming magnesium oxide), which is why it is used in fireworks, flares and flash bulbs. Gemstones get their colours from other elements (e.g. chromium in ruby), not magnesium. Examination of the alimentary canal uses a barium meal (barium sulfate) for X-ray contrast, and water purification uses chlorine or alum — not magnesium.