There is a wealth of established knowledge on the commonly accepted physical and metaphysical uses for most known crystals. These can be lumped into a wider category like, say, color, or become extremely specific like the difference between four subtle subtypes of one stone. These are great places to start when you're building your collection and deciding which stones to use for which situations.
Hibiscus Moon has some awesome methods for personalizing your stones' uses based on several different scientific and metaphysical theories. There's a little something for everyone.
I've combined methods that have worked for me in the past and what I've learned studying crystal healing thus far to really hone in on how to be as efficient as possible. While the bedrock of my practice is in fact science based, choosing my crystals has always relied on intuition and divination from my tarot and oracle decks. This is not the only way. This is simply my way.
Using Tarot (and/or Oracle) Cards to Determine How to Use Your Crystals
1. Retune/cleanse new crystals. Whenever I acquire a new stone, I cleanse it before I add it to my collection and use it. This is not necessary, scientifically speaking. Crystals don't absorb or repel or transfer "good" or "bad" energy. They have a set vibration or frequency, just like every other piece of matter in the universe. BUT, our ability to entrain with them and effectively use them may be momentarily marred if we perceive a stone to be "unclean" or perhaps dingy with unknown intentions or vibes. Look, our minds are incredibly powerful. Our perceptions literally create our realities. If you feel weird about not knowing who handled your crystals before you, there's no harm in performing whatever ritual you want to get yourself to a blank slate. For me, this is something I need to do.
I like to burn sage and run the crystals through the smoke and then play my singing bowl for a few minutes. That's it. Now they're ready for action.
2. Meditate. I spend some time (5-20 minutes) sitting with each crystal. I hold it, look at it, smell it. I meditate with each one and try to be aware of anything that comes through into my thoughts. Make note. How do you feel physically? How do you feel emotionally? Do you have any specific words or sounds pushing forward?
3. Immersion. I keep notes for a few days (usually 4) while I go about my usual routines with my crystal within about 5 feet at all times. The easiest thing to do is just toss the thing in your pocket and go. Be as aware as you can. What signs do you see? Any recurring themes or animals or situations? Sometimes this process is challenging because I'll get an increased awareness around one or more chakras which basically translates to mild pain or discomfort. You may be extra sensitive to certain stones and choose not to hang out with them 24/7...that's okay! Just make a note about it.
4. Conduct a card reading. Perform a simple reading using whichever deck calls to you. Sometimes I use a tarot or oracle deck exclusively, sometimes I do a mixed reading. This is always led by intuition, so do whatever feels right to you. Focus on what that stone means to you, what it can best be used to bring you the most efficient transformations and highest good. I draw 1 or 2 cards to help focus the message.
5. Cross-check your resources. Finally, I like to check out the established knowledge base about that crystal. What do the books, blog, and experts say? I think you'll find that your findings and their findings will overlap, but don't worry if they don't! Stick to what makes sense for you.
Regardless of your findings, please don't ever ingest or immerse yourself in a crystal bath without checking and double-checking the toxicity of that stone. A non-exhaustive list can be found here.