If anyone here has metal detected for gold nuggets, I'm sure they've walked right over fluorescent rock and mineral specimens without even knowing it. It's estimated that 15% of all rock and minerals are fluorescent so that gives you some idea how common (or uncommon) they are.
The most common are calcite and chalcedony which fluorese red and green under SW lamp respectively. But there's about 500 other rocks and minerals that fluorese yellow, purple, blue orange, pink, etc. But not all fluorese in SW.
Value is determined by aesthetics, size of specimen and the number of colors or combos. A single color specimen would normally be worth less than a two colored one unless it was a rare species. Generally speaking the more colors, the higher the valuation but a lot of other factors can go into it.
It's estimated there's well over 10,000 collectors world-wide with the majority of them here in the USA so there's a good market for specimens.
All that's required is a good SW field lamp as that's the most common wavelength...254nm. LW is 365nm and midwave I believe is either 302nm or 310nm.
If I was going to recommend a SW field lamp this is the one I would get.
It can be used at night and on cloudy days. It's a little expensive but all it takes is one good specimen and it's paid for.
Most of you guys know where the local mines, tailing piles, outcroppings, etc are and those would be the obvious places to check first. But there's a lot of other areas where they could be found as well. All you have to do is search for them...