The area around San Juan del Río is one of Mexico's principal opal-producing regions, known particularly for fire opals — translucent stones in shades of orange, gold, and red, sometimes with the shifting play of colour called "fire." Several workshops in and around the city offer tours where you can watch stones being extracted and polished, and purchase directly at prices well below what you would pay in Mexico City or tourist towns.
The opal shops along the streets near the main plaza sell cut and uncut stones, simple settings, and more elaborate jewellery. For mine visits, ask at your accommodation or at the tourist office near the plaza — guided visits to active mines can be arranged and typically include an explanation of the geology and extraction process. Bring cash; most opal sellers prefer it.