What color are calcium carbonate crystals typically?

Calcium carbonate crystals are typically found in a range of colors, including being colorless to yellow. This characteristic is due to the natural formation of the crystals, which can sometimes include impurities leading to a slight coloration. The colorless nature of pure calcium carbonate is a result of its crystalline structure, which allows light to pass through without much scattering. The presence of slight impurities can impart yellow hues, but this is not a defining characteristic of calcium carbonate itself.

The other colors presented in the options do not accurately reflect the typical appearance of calcium carbonate crystals. Dark brown to black, greenish, or bright blue are not common colors for these specific crystals due to their unique chemical composition and crystallization process. Thus, the range of colorless to yellow is the most accurate depiction of calcium carbonate crystals.