Our crystals go through a lot; we use them in ritual, in healing, in divination, in all sorts of things. I keep a few small ones in a pouch on me all the time, just in case I need some extra energy for something, be it a headache, indigestion, or concentration.
With all that they go through it's not surprising that they will pick up some dirty energy. This is why every now and then, depending of how often they are used, you should cleanse them. So here are a few methods that I like to use.
Firstly, there is brown rice. You can put your crystals on a tray, in a bowl, or in a small container filled with brown rice. You can then leave them in there for a few nights, or if u like, put them outside underneath the moonlight on an evening when it is a full moon. I like to put mine on a silver tray and leave them out on the full moon.
Another way to cleanse is to use sea water, or salt water. Do the same as with the brown rice. But be careful. Salt water can be corrosive to certain crystals, so I tend to stay away from this technique with my clusters.
A very powerful method of cleansing, but also very risky, is to bury them. Bury your crystals and leave them there from full moon to full moon. But also try not to do this with cluster, they might break when you dig them up again. I say this is dangerous because you never know if you will remember exactly where you buried them. Also you might just lose a couple in the process. Recently I did this and when I went to go dig them up, I saw my dogs had beaten me to it. Luckily they didn't dig very deep and I still managed to recover all of them, I think.
When keeping your crystals in close proximity to each other, they can inadvertently mess with each others energies. A way to prevent this is to place a few amethysts amongst them, or keep them on a tray or in a bowl of brown rice.
These are just a few techniques to cleanse your crystals, there are many more. Find what works for you and then use it.