Tridymite

Tridymite is the high-temperature, low-pressure polymorph of quartz, typically found as thin pseudo-hexagonal (triclinic) crystals within certain igneous rocks. It was first described in 1868, with the type locality in Hidalgo, Mexico. The surface of this specimen is covered in excellent micros, making it a valuable piece for collectors of rare minerals. Tridymite is notably scarce on the market, and finding specimens of this polymorph is a great opportunity for collectors looking to add something unusual to their collections.

Interesting note: In December 2015, NASA's Mars Science Laboratory team made an unexpected discovery: high concentrations of tridymite in a sedimentary mudstone located in Gale Crater on Mars. This discovery was surprising due to the rarity of tridymite on Earth, where it is typically associated with silicic volcanism—high-temperature, high-silica magmas. The presence of tridymite on Mars marks the first in situ mineralogical evidence of Martian silicic volcanism. This finding adds to the growing evidence of the complexity of igneous processes on Mars, suggesting that the planet has experienced igneous evolution that includes high-silica magmatic compositions.*

*https://www.pnas.org/doi/10.1073/pnas.1607098113