Claystone is a mud rock that is entirely (or dominantly) clay-sized material.
Claystone is usually massive, and is very soft and can be easily broken with hand pressure.
It will crumble into irregular pieces rather than fissile fragments.
Shale is a mudstone or claystone - AND shale is ‘shaly’. That is, it is fissile or platy, and breaks into very thin fragile sheets. Shale is distinguished from other sedimentary rocks because it is fissile and laminated.
"Laminated" means that the rock is made up of many thin layers.
"Fissile" means that the rock readily splits into thin pieces along the laminations.
The only difference between Mudstone/Claystone and Shale is that mudstones break into blocky pieces whereas shales break into thin chips with roughly parallel tops and bottoms. Both are made of ancient mud.
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