Alabaster is famous as a milky white mineral that can be translucent if carved thin enough. There are two types of alabaster. One is gypsum, which is used today. The other is calcite, which was used in ancient times including by the Ancient Egyptians. The two kinds share the same white appearance, but are distinguished from each other due to a difference in hardness. The gypsum kind has a Mohs hardness rating of 1.5-2 and be scratched with a fingernail. The calcite kind has a Mohs rating of 3. It can’t be scratched with a fingernail but can be scratched by a knife Hydrochloric acid makes the calcite alabaster fizz and bubble, but does not effect gypsum alabaster.