What color is stalagmite?

Primary colour: white. Calcite itself is transparent but in mass the crystals give a white effect in the same way as sugar or snow. A "straw" (in US caving, "soda straw") stalactite is so thin it is nearly transparent.

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Mineral (metallic) impurities naturally present within the source limestone often tint stalagmites and their fellow "speleothems, usually to a buff or yellowy-cream colour, but shades of brown or red (from iron) or blue (from copper) are known, although rare.