Believed to have once been the part of the core of a long vanished planet or asteroid, these are thought to have come from the large cluster of masses between Mars and Jupiter called "The Asteroid Belt." These are some of the densest materials on the planet and are strongly magnetized so will they will stick to basic magnets when brought together. Since these meteorites are so dense, they are much heavier than normal rocks found here on Earth.
These alloys grow into complex, interlocking, crystalline patterns called a Widmanstatten pattern or also called Thomson Structures. Most of these structures are only seen once a meteorite is cut and etched with a solution of nitric acid. Once the crystals are revealed the size and bandwidth of these are measures and separate them into different sub-categories or classes.
Most specimens in this group are composed of 90-95% iron with the rest being made of trace elements and nickel. The classes of this meteorite are separated by both structure and chemical makeup. The two primary substances that help separate them into these classes are kamacite and taenite.