I have always had a fascination with gemstones and minerals - opals, amethysts, crystals, you name it. Gladly also fancier stuff, like rubies and diamonds. There are all kinds of little shops out there selling that, I've noticed, but would that work, as a primary money earner? Let's say, in a town of 120.000 people, with plenty of tourists and students, and a healthy arts scene? I suppose it should also carry stuff like meditation music CDs, etc.
Do shops like that represent a worthwhile undertaking?
"worthwhile" is a value judgment which can be defined in 2 basic ways: enjoyment and remuneration. Clearly you would derive enjoyment, especially if you like the artsy crowd who would be into crystals for spiritual reasons. Finding out if such shops make money should be easy, many of the existing businesses are publicly held corporations so there will be public records of their basic finances. Also you can check to see what such stores have sold for recently and that will give you an idea of their profits. I would think that type of business would be all about atmosphere to attract the type of customers you want.
One thing you should consider; the U.S. 'dollar' (Federal Reserve bank note) is about to go through some unpleasant changes, some time in the next few years. Since it is the world's "reserve currency" this will affect things like gems, tourism, food and energy. In other words, when the dollar hyper-inflates or collapses, a gem shop would be very much affected. This might be good or bad for such a shop. Bad when tourism drops off, good if you can cater to people wishing for a more secure store of value than paper promises of a faith-based currency.