Garnet is such a complicated set of stones, we are going to link you to the GIA page on garnets.
"Garnets are a set of closely related minerals that form a group, resulting in gemstones in almost every color. Red garnets have a long history, but modern gem buyers can pick from a rich palette of garnet colors: greens, oranges, pinkish oranges, deeply saturated purplish reds, and even some blues.
Red garnet is one of the most common and widespread of gems, found in metamorphic rocks (which are rocks altered by heat and pressure) on every continent. But not all garnets are as abundant as the red ones. A green garnet, tsavorite, also occurs in metamorphic rocks, but it’s rarer because it needs unusual rock chemistries and special conditions to form.
Demantoid is a rare and famous green garnet, spessartine (also called spessarite) is an orange garnet, and rhodolite is a beautiful purple-red garnet. Garnets can even exhibit the color-change phenomenon similar to the rare gemstone alexandrite.
All garnets have essentially the same crystal structure, but they vary in chemical composition. There are more than twenty garnet categories, called species, but only five are commercially important as gems. Those five are pyrope, almandine (also called almandite), spessartine, grossular (grossularite), and andradite. A sixth, uvarovite, is a green garnet that usually occurs as crystals too small to cut. It’s sometimes set as clusters in jewelry. Many garnets are chemical mixtures of two or more garnet species."