Scoria!

SCORIA

it is a textual term for macrovesicular volcanic rocks ejecta. it is commonly , but not exclusively basaltic in composition. scoria is light as a result of numerous macroscopic ellipsoidal vesicles but most scoria has a specific gravity than 1 & sinks in water. the vasicularity results from the exsolution of magnetic volatiles prior to chilling. scoria differs from purnice in having layer vesicles & thicker vesicles walls, & hence is typically darker in color. it is generally dark brown, dark red, dark black. the word comes from greek skoria means "rust". as raising magma encounters lower pressures, dissolved gasses are able to exsolve & form vesicles. some of the vesicules are trapped when the magma chills & solidifies. vesicules are usually small, spheroidal & donot impringe upon one another, it is formed from a thin layer of froth occuring on some basaltic lava flows due to the bursting of vasicle walls. the thin glass threads are the inter-section of burst vehicles. this is the lightest rock on earth with its specific gravity less than 0.3.