What is the difference between sandstone and obsidian?

Sandstone is sedimentory. It also often red to brown, light gray to nearly white. Sometimes it is yellow or green. It usually is composed of rounded grains that are all of the same size; and it is usually medium grained. Some sandstones show slight color variations. How Was It Formed: quartz sand that is produced by the weathering of other rocks (such as granite, gneiss, and other sandstones) is deposited by rivers, waves, or wind. The sediment may have been a sand bar, an ocean beach, or desert sand dunes. The sand is buried under other sediments, compacted by the weight of those sediments, and cemented by material dissolved in water that seeps through it.

Obsidian is igneos. Obsidian is a glass and is usually black, although sometimes it may be slightly grayish or greenish. It may include some white crystals that look like snowflakes (snowflake obsidian). It may include swirls of a red color. Obsidian breaks and chips like glass. The location where the chip came out is scoop shaped, like the inside of a clam shell. The chip often has ridges that that are semicircular. This kind of break is called a conchoidal fracture.