Some are stone, some iron, some both. It can be hard to tell.
Irons come smooth, black, or rust. stone has a wide range, most are moving fast, if they survive to the surface, they can have a burned crust, but many come in very slow, and are deep cold if found quickly. Every few years comes a story of one going through a roof, or car.
Some things that look like them are volcanic bombs, blown out of a volcano, some time in the last few billion years, hundreds of miles from the volcano. Volcanos erode quickly, so there may be no trace.
All along the Gulf Coast are bits of glass formed when the big one struck the Bay of Campeche 700 miles away.
Most vaporize in the air, or striking the ground. Normal cosmic speeds run 40,000 miles per second.
A big one reaches the surface about every 25,000 years, with the force of a hundred atomic bombs.
The crater in Arizona was formed 50,000 years ago, the next hit Mauritania, 25,000 years ago, and 4 out of five hit the ocean. Several cultures Sumarian, Mayan, said that a powerful god destroys the earth every 25,000 years. Sumarians called it Marduk, the Mayans Quetsacotal, the flaming feathered serpent in the sky.
In our words, they spoke of a monster of a long period comet, which is due, The Mayan long count ends in 2013, end of the Fifth Age. Two known smaller comets left the Leonid, and Persid metor showers, left so much debris that thousands per hour streak in the sky when the earth goes through their path to the sun.
Craters survive in deserts, but there were likely many more, something like the Earth in Cowboy Bebop.