The mineral calcite consists of calcium carbonate CaCO3. It is a very common mineral that makes up limestone. Beginning students in geology labs quickly learn that calcite can be easily identified because it fizzes in weak hydrochloric acid.
Also, if the crystal of calcite is clear (transparent), any writing underneath of it is "doubled" when viewed. This effect is shown in the first two images below. The first image is of a small, clear crystal, about 3 cm across. It is sitting on a white piece of paper, with the letter "X" written on it. Light is refracted through the crystal and produces the "doubling" effect (called double refraction). Single perfect crystals like this one, have what is known as the rhombohedron shape (a six-sided prism).
The second image is of a longer (7.5 cm) clear rhombohedron crystal, sitting on a white piece of paper with the word "calcite" written on it.
There are also other colored varieties of calcite, as shown in the following "color wheel." For scale, the green crystal is 4 cm length.