unknown sedimentary rock

This rock is made up of very fine crystals that can't be viewed even with a hand lens. The rock demonstrates numerous sharp edges and scoop shaped depressions on all sides. While this sample is dull grey to tan colored this rock's colors vary widely and it can exhibit red, black, or white-ish coloration. The fine crystals making this rock are fairly hard and will scratch a glass plate. There are no fossils visible in this rock. The rock doesn’t taste salty at all. rocksandminerals CC BY(4.0) via sketchfab [https://creativecommons.org/licenses/by/4.0/legalcode]