Reddit mentions of Colorado Rockhounding: A Guide to Minerals, Gemstones, and Fossils (Rock Collecting)

You are in for a damn treat -- Colorado rockhounding is some of the best anywhere! I'll give you my abbreviated list of stuff in South-Central CO, a couple hours away from Denver, since that's the area I'm most familiar with. But there is plenty of stuff directly in your neck of the woods, and I'd highly recommend picking up copies of Voynick's Colorado Rockhounding, Kapelle's Rockhounding Colorado, and especially Mitchell's Gem Trails of Colorado. The Roadside Geology book is good, too.

Trout Creek Pass pegmatites, about 20 minutes east of Buena Vista. The Clora May Mine is the easiest to access, with only a ~15 minute hike uphill. It's got some great chunks of pink feldspar, biotite and muscovite mica, and some massive (in the geology sense of the term) quartz/smoky quartz. There's a smaller amount of black tourmaline, although the quality isn't generally great IMHO. But the real prize is the uncommon rare earth minerals such as Aeschynite-(Y), Polycrase-(Y), and Xenotime. They're generally not breathtakingly beautiful, but a fantastic addition to your collection if you're a chemistry buff. Don't try and take them on the plane with you, though -- some are surprisingly radioactive.

Ruby Mountain, ~20 minutes south of BV. It's easy to find, and not bad of a hike up. Halfway up, there's a good outcropping for perlite and obsidian "Apache tears". The top is mostly rhyolite, but there are small (think millimeters, not centimeters) garnets in some pieces. More rarely, there are some funky "flower" shaped formations with tiny crystals of topaz. This site is pretty well picked over and half is on private land, but it's definitely convenient.

The Rock Doc. Awesome rock and mineral store in the same area as the above. You can also pick up supplies, books, and local intel on collecting sites.

Also not a site as such, but the National Mining Hall of Fame in Leadville has a fantastic collection. The Rock Hut mineral store in town has good specimens, but a bit pricey for my taste.

Mount Antero. The aquamarine is legendary, but this is definitely not a casual collecting spot. Inaccessible most of the year, lots of active claims, bad afternoon lightning storms. Wouldn't recommend unless you happen to plan ahead with an experienced local as a guide.

There are a few quarries, mines, and collecting sites around and behind Salida. Midway Springs has some cool Aragonite features -- you can find bright red-and-white chunks that look like miniature caves, including speleothems. There's good Magnetite and Actinolite specimens from the Calumet Mine, but it's not always open to collecting. I've never found anything worth writing home about in this area, but there's many sites I haven't seen.

Browns Canyon. The old fluorite mine on the way down has a bit of botryoidal white/clear fluorite on the dumps, but as above -- nothing to write home about. Cool igneous rocks further down the road, and I've found hydrothermal deposits of the uncommon tennis-ball green Vanadium mineral Volborthite.

There's stuff up towards the ghost town of St Elmo, but it's a bit hard to get to (especially before June/July) and there are a lot of active claims in the area.

Go to the Bayou Salado Trading Post in Hartsel. The owners hold claims on nearby land for blue and yellow Barite, and you can dig for a nominal fee of like ~$5. It's very hot in the summer, but the crystals are abundant and just lie scattered in the red mud. They actually get more blue with sunlight exposure, unlike most minerals! There are some bigger clusters buried, if you're willing to dig.

Bonanza is an old mining town (pop. 16) further south in Saguache County. Not a lot to find without local help and 4WD, but I have found tiny crystals of Sylvanite, one of Colorado's famous Telluride minerals.

Westcliffe / Silver Cliff, previously a silver mining region. I've only been down this way once, but there are lots of digsites on public land north of town. My best find from that day was Rhyolite with Liesegang Rings. The Sangre de Cristo Gallery and Rockshop in town had great prices, in my experience.

Pinnacle Mine, across Monarch Pass. It's an active uranium mine and closed to collecting, but I thought I'd mention it because the front gate (covered in "No Trespassing" and "Danger -- Radioactivity" signs) is an awesome place for a selfie.

Also take a look at the Peak to Peak online guide to CO sites, GatorGirl's page for Colorado and the Mindat page.

And don't forget to drop by the Denver natural history museum's mineral hall! The Alma King is pretty much the iconic piece of Colorado mining history!