A list of all precious and semi-precious stones is available at GemSelect.com. Another list, which includes artificial and organic gemstones, is available on Wikipedia. Each entry in the Wikipedia list links to an article about the gem that describes natural variations in color and appearance.
In the most technical sense, the word gemstone refers only to pieces of mineral crystal in various colors that are used to make jewelry and other ornaments. However, many other substances frequently are used for similar purposes and are commonly referred to as gemstones. These include rocks such as lapis lazuli and unakite, inorganic compounds such as opal and obsidian, organic substances such as amber and pearl synthetic crystals such as cubic zirconia and high-lead glass.