So in honor of #TheClubsRockingNewYear challenge now is my time to shine - I did Geology as my degree and of course have ended up making games for the last 20 years instead of being a world renowned Geologist... anyway and here is a very short thread on the types of rocks.
Ignious Rocks - formed from the cooling of magma (the hot sticky stuff the explodes out of volcanoes - although technically it's called lava when that happens) either on the surface or under the earth's surface. An example of an ignious rock is granite - used in kitchen worktops. 
Metamorphic Rocks - formed by extreme heat or pressure under the earths surface. Examples include Marble and Slate
Sedimentary - these are built up of multiple layers laying on each other such as sand (giving sandstones) and mud, or organic sediments such as small animal shells (giving chalk).
Igniouis are typically the hardest rocks with sedimentary being typically the weakest - you'll be able to pick off bits of chalk or sandstone from a cliff for example.
If a rock is exposed on the surface and it looks fairly intact it's probably an Ignious rock as they can resist the elements more than a seimentary - the Grand Canyon is caused by the river carving through the top sedimentary rock layers - with stronger metamorphic at the base.
Cracks in Ignious rocks usually are the result of water getting in and freezing and expanding in voids in the rock.
Anyway a small walk down memory lane of what basic geology information I have retained over the years - some may or may not find useful - I'm going back to the rock sculpting now. Cheerio! :-D
Oooh one other things in regards to sedimentary rocks - sediment obviously get laid one layer on top of each horizontaly - so if you see obvious layers pointing in any other direction than flat it means they've been moved into position by geological processes.
And another thing - Ignious rocks tend to fracture and provide harder angles, it generally takes lot of weathering (water or wind) to erode them into soft shapes, Metamorphic rocks are slightly softer in comparrison and sedimentary are very easily eroded into soft, rounder shapes
Damn my fat fingers with this thread - it's of course Igneous not any other spelling. 🙃
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