What substance is significantly abundant on the moon but rare on Earth?

Natural glass, specifically lunar glass formed from volcanic activity or impact events, is significantly abundant on the Moon due to its geological processes. The Moon's lack of atmosphere means that when meteoroids strike its surface, they can create high temperatures and pressures, resulting in the formation of glass from the melted surface materials.

On Earth, while natural glass can occur, it is much less common for several reasons. Earth’s atmosphere, weathering processes, and active tectonic movements tend to destroy or alter many geological features, including those that could produce significant amounts of natural glass.

In contrast, KREEP, which stands for potassium, rare earth elements, and phosphorus, is associated with certain lunar rocks but is not as universally abundant as natural glass. Basalt is also present on both the Moon and Earth, typically as a result of volcanic activity, hence it is not unique to one body over the other. Comets are primarily composed of ice and dust, rather than being a substance that exists abundantly in one location versus another.

This context helps clarify why natural glass is the choice highlighting its unique prevalence on the Moon compared to Earth.