This geocoin designed by Dutch geocacher Technetium is a functional circular slide rule. It can be used to multiply or divide. The slide rule also has a trigonometric scale for the calculation of sine, cosine, etc.
To use, line up the 1 on the outer index of the inner disk with a number on the outer disk (the first operand). Find the second operand on the inner index and it will line up with the product on the outer disk.
For example, line the 1 up with 2.5 and the 3 will line up with 7.5 (2.5 x 3 = 7.5). The 5 will line up with 1.25 which can be regarded as 12.5 because on the circle the numbers will continue infinitely.
That infinite progression is the reason the inner circle contains a fractal -- Sierpiński's triangle. The largest empty triangle contains the electron configuration scheme of the element technetium. In the other triangles rays are visible: No stable version of technetium exists.
The back of the coins shows a Smith diagram. This is used by electrical and electronics engineers to determine impedance and related things.