This microcrystalline material generally occurs at the base of bedrock layers created by lava flows. Hornfels is created as molten lava flows over the earth’s surface: molten rock rapidly cools and solidifies upon contact with surficial ground materials. This rapid cooling and solidification means there is no time for large crystals to form in this contact zone. The result is a glassy substance that fractures similarly to obsidian and flint; it is therefore an excellent material for stone tools.