The words refer to the mineral content of the rocks. Where to look for igneous rocks The best place to look for interesting rocks, minerals or crystals, is the place where the intrusion meets the original rock. That is why you will hear rockhounds frequently talk about a pegmatite dyke . This is where the hot lava contacted the original rock. There is one texture of rock called vesicular and sometimes referred to as frozen froth.
Pumice is a lava rock that is hardened foam.
When larger bubbles became trapped in the hardened lava, they formed pockets and over many millions of years the pockets filled with other liquids that formed mineral crystals. The fillings are called amygdales , a Greek word that means almonds. The rocks are known as amygdaloidal rocks. Banded agate forms in large gas bubble cavities. Geodes also develop in these pockets.