Lava stone is a dark-colored igneous rock with abundant round cavities in the shape of bubbles called vesicles. Its color ranges from black or dark gray to deep reddish brown. Lava stone generally has a composition similar to basalt, but it can also have a composition similar to andesite.
Lava stones form when magma containing abundant dissolved gas flows from a volcano or is blown out during an eruption. When molten rock emerges from the Earth, the pressure on it is reduced and the dissolved gas begins to escape in the form of bubbles. If the molten rock solidifies before the gas escapes, the bubbles become small, rounded or elongated cavities in the rock. This dark-colored igneous rock, in which bubbles are trapped, is called slag.
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