For the first lesson regarding crystals, we're talking about crystals that are easy to find or purchase. For instance, have you ever gone to a tourist trap and seen those crystals sitting in there, most of them dyed, some of them not? Yeah, that's what we're talking about.
Let's get the dyed crystals out of the way first. While it normally wrecks the crystal when you try to clean the dye out, it's still possible to use dyed crystals for their original purpose (the most commonly dyed crystals, in my experience, are tree agate or quartz, but I have seen dyed moonstone). It's also possible to use them in color magic, as they usually get dyed into colors like pink, green, blue, and purple.
If you do some digging, you can find some undyed crystals too - namely red jasper, tree agate, rose/clear/snow quartz, green aventurine, amethyst, and even citrine. These stones are also good beginner crystals and can all be used for many things. You may have noticed that all of these, except for the agate, are a type of quartz. Quartz of most sorts is really common and easy to get a hold of. None of these are dangerous to put in water, but the quartz, amethyst, and citrine all will fade with prolonged exposure to sunlight.
Amethyst is a good stone for all-around healing. It helps with gender dysphoria, calms the mind, aids meditation, guards against psychic attack, and much more. It can even be used for cleansing, though it can't cleanse itself.
Its cousin citrine is good for attracting wealth and abundance, banishing negative energy, boosting self-esteem and self-confidence, enhances concentration, boosting energy, and cleansing. It's one of the few stones that never needs cleansing because it cleanses itself.
Aventurine comes in many colors, but green is the one you'll see most often. Green aventurine is your standard prosperity and abundance stone, but it also helps with healing and protecting one's heart. It also helps with nausea and dissolves negative emotions and thoughts. Like amethyst, it helps with calming.
Red jasper, meanwhile, stalks me. It's why I use the name Jasper as my craft name, actually. But enough about me. Red jasper is a protective stone, but it also helps with grounding and brings problems to light before they become too big. It's a good "worry bead" because it helps calm emotions. It's got a lot more properties, but this is a basic guide.
Tree agate is a very common kind of agate and can be confused for many other types. In addition to abundance (a very standard thing with green stones), it's also associated with good luck and protects from energy vampires. Like amethyst and aventurine, it also helps with calming, among more things.
We finally get to rose quartz. Rose quartz is the stone of unconditional love, and that phrase is basically "the mitochondria is the powerhouse of the cell" except for witches. It strengthens empathy and sensitivity, draws off negative energy, transmutes emotional conditioning that no longer serves, and soothes internalized pain. There's a lot more than that, all focused on emotions and the heart.
Clear quartz is one of those crystals that can replace most other crystals when properly charged. It can amplify, store, absorb, release, and regulate energy. It helps with balancing and harmonizing "subtle bodies" (whatever that means to you).
As previously (and often) mentioned, you're bound to find more than that depending on the tourist trap, but those are the normal ones you'd end up finding.