A tunnel is characterized by a hole that goes through. This is partly the case with this piece of jewelry, so whether it is a tunnel or not can be discussed. If you choose one of the small sizes, from 2g (6 mm) up to and including 1/2" (12 mm), there is no hole so it can be considered a plug. But all the bigger sizes are see-through, so they are tunnels :)

In the sizes that range from 2g (6 mm) up to 1/2" (12 mm), the front part has the same diameter, 5/8" (16 mm). If you check out the pictures above with all tunnels, you will see that four of them have the same front. The part that goes through the ear, however, has the diameter of the gauge size you select. 

The tunnel is made of surgical steel, and the front is crafted from brass. The front part is a compound of special gears. You will find yin and yang and several small gears and screws with a design that is categorized as steampunk.