text: Due to this rock's metastability, no specimens older than the Cretaceous era have been observed.
full_question: Due to this rock's metastability, no specimens older than the Cretaceous era have been observed. Once the past temperature of this rock is known, the SIMS-SS profile can be used to determine its age because this rock absorbs water at a constant rate. The appearance of the "fire" variety of this rock is due to nanoparticles of magnetite, while the appearance of its "snowflake" variety is due to gray spherulites centered on locations where crystallization has begun. This rock is commonly formed by the cooling of relatively viscous, silica-rich granitic magmas; because of the rapidity of cooling, it exhibits few to no crystals. This rock's conchoidal fracture made it ideal for use in surgical blades and stone tools. For 10 points, name this usually-black extrusive igneous rock, a volcanic glass.
first_sentence: Due to this rock's metastability, no specimens older than the Cretaceous era have been observed.
raw_answer: obsidian