Slate is a fine-grained metamorphic rock. It's main constituents are quartz, chlorite, mica & calcite. Slate is formed when ocean or riverbed sediments are compressed and heated by the earth's crust.
A characteristic of natural stone is that there may be extreme variation in color and veining from tile to tile. This characteristic is common in many types of stone, and is part of the inherent beauty of using a natural product in your home.