When it comes to apps to aid you with your rockhounding adventures it really breaks down to three categories: geological, navigation, and overland travel / camping.
Hands down the best free app out there to aid you in your understanding of the geology around you is the Rockd app which is free on both Apple and Android.
The Rockd app gives you a mobile friendly version of the Macrostrat site which offers geologic maps from around the world and at numerous scales that have been homogenized into a single database. As you zoom in and out of the map, scale-appropriate geologic maps are shown, and clicking on the map reveals primary data about each unit.
The app also has some social feature like check-ins which is nice to use to see some of the spots in your area that other people have found interesting.
The only real downside to using the Rockd app is that it requires a data connection to load the maps which can be a problem in the field with a limited connection.
Without a doubt the most useful app you can get to aid you in your rockhounding adventures is GaiaGPS. This is something that I have covered on my YouTube channel in great depth.
One of the things that makes Gaia GPS so amazing is the ability to use it across a number of devices such as your phone, laptop, and tablet and sync them all together so when you sit your computer in the morning planning a trip, that route can go straight to your phone for later use.
I highly recommend the paid premium membership of $40 a year (May 2021) as it gives so many features that are extremely helpful when rockhounding. However I am an affiliate with GaiaGPS so you can now receive a discount on Gaia by using this reference link https://www.gaiagps.com/discounts/?fp_ref=currentlyrockhounding
Some of my favorite layers are the public lands, private lands, snow pack, geo bedrock, mines, and satellite / topo hybrid.
All of these layer are all able to be downloaded to your phone or tablet allowing offline back-country navigation.
Couple that with ability to drop pins, and build custom routes to follow and you have a very powerful tool for navigation, land ownership and weather.
Overland Travel & Camping:
When it comes to the subject of long distant overland travel, road trips, and looking for places to camp you really only need two apps to help you with these subject The Dyrt and Overland Bound.
Both apps / sites have extremely helpful communities built around them with so much to offer that it would be hard to fully capture it here.
I highly recommend both of them if you plan on doing extended travel.