Some tattoos, usually those done privately or with traditional materials, can use a form of pen ink. They can also use natural materials such as soot, earth or even blood. However, most commercial tattoos use different kinds of ink to those used in a pen.

Tattoo ink is based on pigments - substances that give it colour. A number of metals are used including cobalt, zinc, lead, mercury and many others. Some of the pigments used in tattooing are so strong that in some countries, eg the US, their use is monitored and regulated by a government agency. To give you some idea: some of the ingredients used in tattoo ink are strong enough to be used in car paint.

Even though tattoo ink is so powerful, in the last few years new inks have been developed which are supposed to be easier to remove. All the same, I would be careful about choosing a tattoo, just in case it wouldn't come pff later.