The first group of foliated rocks is call slates . Slate splits into smooth slabs. Gneiss is another group of metamorphic rocks. It has a streaky appearance due to alternating layers of minerals. Non-foliated metamorphic rocks include the group marble .
Marble can be fine or coarse.
It can also be scratched by a knife and bubbles when put into acid. The last group of metamorphic rocks is called quartzite . Quartzite is the hardest and most resistant rock. All three types of rocks form the rock cycle because anyone of the three can help to form either of the other two groups. This is illustrated by the cycle given below.