Calcite is a very interesting mineral. In addition to forming in cubes and rectangular prisms, many kinds of calcite are transparent, meaning you can see through them. Iceland spar calcite is actually used in some microscopes! Calcite is fairly soft and can be scratched by a penny. It is closely related to chalk and is a big part of limestone and marble. Just like limestone, calcite reacts with acids. Vinegar, which is diluted acetic acid, will cause it to bubble from the chemical reaction. Calcite is used in things such as toothpaste, antacids, chewing gum, glue and soap. It is usually white, colorless, or tinted yellow.