Obsidian is a glassy, lava-made rock. Shale, marble, slate, granite and petrified wood contain crystals that can be seen with a magnifying glass. Granite, rhyolite and felsite are not similar in nature and cannot be single crystals but are crystalline. Salt is a clear, brittle mineral that is used for flavoring food and preserving food since ancient times. Salt is used in a manufacture of a large number of chemicals.
Salt consists of sodium and chlorine.
Salt forms crystals that are almost perfect cubes. Salt can be broken down to make a variety of sodium and chlorine products. Salts are among the most important chemicals. A crystallographer is a scientist who studies the atoms in crystals and crystals themselves. Crystallography is very important in physics, physical chemistry and biochemistry.