Density of pumice and obsidian. Beaker of water containing a sample of pumice (floating, top) and obsidian (black), demonstrating the relative difference in their density. The two rocks are both types of extrusive igneous rock, formed by the rapid cooling of lava. They have a similar composition, both having a high silica (silicon dioxide) content. The difference in their density arises from the difference of their textures. Pumice is porous, having a vesicular texture (containing many holes). It forms from lava that contains gas bubbles. Obsidian is glassy because of the rapid rate at which it cooled.