Just about any rock can be metamorphosed like say sandstone which then becomes quartzite. Its more noticeable in effect in sedimentary rocks only because those are less dense to begin with. With igneous rocks it gets more interesting.

Shale is shale because it is sedimentary and has a certain partical size. You can go through a whole range of siltstone and mudstone, etc depending on the size of the particles comprising it. Those of you who live in N.Y State go up to Wurtzboro and you will find an old abandoned railroad Tunnel. Its about 1 mile long and the mountain is solid slate there for the taking. (just be careful a chunk doesn't fall from the roof on you) Slate has perfect cleavage so that's why its been used as a roofing tile for a long long time. A few strikes and you can get great flat pieces.

In general rocks and minerals are usually found in certain associations. Most minerals that form as xtls are from the igneous state where as the rocks cool from the liquid state and if they cool slowly enough and have room xtls will form. The forces governing the formation of the mineral will determine the type of xtl you get consistant with the xtl system that mineral falls in. Though its not a common association you can have garnets in a sedimentary rock but it would have to be a very special circumstance. For instance the garnet I had previously presented here is formed in a schist which is basically rock with alot of mica in it and the garnets over time will weather out of the schist and just be loose. Most times they will just either lie where they are and weather away of be carried else where and do the same but they can also be washed somewhere so they can find themselves in newly formed sedimentary rock over time of course. Of course this is theoretical and I don't know if such deposits do exists but I would imagine they do.

Like what Howard said that's why I'm going to experiment with that chunk of beryl I have.