Opal - Andalusia Mine, California
Contributed by: FMS Admin
Date: May 9th, 2026
Locality: Andalusia Mine, Knoxville, Knoxville Mining District, Napa County, California, USA (See on Mindat)
Description:
My then eight-year-old daughter and I explored the old Andalusia Mine tailings for fluorescents. Never before had we found so many interesting and beautiful rocks that had absolutely no fluorescence. Ironically, right next to where we parked the car was an odd boulder. We hit it with our lamp, and it glowed. I cracked it up so I could get it in the car and cracked further when we got home. This is by far the best chuck. Experts have looked at it and say it is opal. That said, I think there is some chalcedony mixed in. Just love the material, especially as no one else has anything like it.
Originally posted by Fire Viper on Nature's Rainbows in 2016.
Summary of luminescence responses:
Opal (Mindat) (RRUFF)
- Fluorescence under Shortwave (254nm Lamp/Mercury) UV light: Green