Boron is a relatively rare element on Earth, making up only about 0.001% of the Earth's crust. However, it is quite abundant in the ocean, where it is present in a concentration of about 4.6 ppm. Boron can be found in a variety of minerals, such as borax, tourmaline, and colemanite. It can also be found in some plants, such as spinach, nuts, and legumes.