Foliated rocks are formed under a great amount of relatively equal pressure, where as non-foliated rocks are formed under high temperatures.

Non-foliated rocks, however, lack this visual characteristic. See the image below.

The reason for this is actually pretty intuitive. Foliated look like they’ve been “smashed” together for a reason: they have been. In the case of these rocks, they have been under a great amount of pressure. Each metamorphic rock has a “parent,” called a protolith. It’s the protolith that has undergone metamorphosis.

On the contrary, non-foliated metamorphic rocks aren’t subject to the same degree of pressure. The amount of pressure that a non-foliated rock has pressed upon it could, in fact, equal the amount of pressure that the foliated rock had, but it’s an unequal pressure, so it doesn’t have the sandwich-type aesthetic. The key force behind the creation of the non-foliated rock is temperature.