Vitrophyre is a volcanic rock with larger crystals (phenocrysts) embedded in a glassy groundmass.
Volcanic rock is formed when molten rock (magma), which is produced by a volcano, has cooled and solidified (crystallized). Groundmass of rock is the fine-grained mass of material in which larger grains or crystals are embedded.
References
Vitrophyre Wikipedia(Text) CC BY-SA