Carboniferous describes something that produces or holds carbon, especially in the form of coal. The picture to keep is old plant matter, buried and pressed until it is on the way to coal. Geologists also use Carboniferous as a name for a long stretch of Earth’s past — hundreds of millions of years ago — when huge wet forests grew, died, and later became a great deal of the coal people dig up. That is why the name feels like ‘coal-bearing time’. You will almost never hear this in a café. It belongs to earth science, museums, and geology papers. Though actually, it does not mean ‘made of pure carbon’ the way diamond or graphite is. A carboniferous rock is tied to that coal-making story: it contains coal, or it comes from the age when a lot of coal was laid down. If you only need ‘has carbon in it’, a simpler word will usually do; this one is the coal-and-deep-time word.