If you've lost the steel striker for your firesteel and need to start a fire, don't panic. Just look on the ground for a piece of common, silica-based rock, such as quartzite (generally the easiest to find), quartz, chalcedony, obsidian, agate, chert, or jasper and use it to strike your firesteel instead. You can also use broken glass or ceramic pottery in the same manner (be sure to protect your hands if possible).
If you're by the ocean, look for dead seashells. Here is a video from one of our readers, who was inspired by this article to try a dead clam shell (special thanks to Cattledog for this video). It works amazingly well!
Tip for using a rock
You might get lucky and find a rock that is already sharp, or, you might have to break it up with a larger rock to get a piece with a sharp edge. This same principle would also apply to unbroken glass or ceramic.
The piece of quartzite shown in the photo at the beginning of this article was broken by a bigger rock to get pieces with a sharp edge.
Once you have a sharp piece, just use it like you would use a steel striker to ignite your tinder: