* Composition: Sandstone is formed primarily from sand grains (made of minerals like quartz, feldspar, and rock fragments) cemented together by minerals like calcite, silica, or iron oxide.

* Variable Components: The specific minerals present in sandstone, their proportions, and the cementing material can vary significantly depending on the location and geological conditions where it formed.

* No Fixed Ratio: Unlike a pure substance, sandstone doesn't have a fixed chemical composition. This means that two samples of sandstone can have different proportions of their components.

Therefore, sandstone is a heterogeneous mixture of different minerals and compounds, rather than a pure substance.