Quartz is composed primarily of silicon dioxide, which is a very stable compound that does not readily react with acids. On the other hand, calcite is composed primarily of calcium carbonate, which is a relatively soft mineral that reacts easily with acids. When calcite is exposed to acids, such as hydrochloric acid or sulfuric acid, it undergoes a chemical reaction that produces carbon dioxide gas and water. This reaction is commonly used in laboratories and industries to test for the presence of carbonates in a substance.