Many methods exist to cleanse and recharge crystals: Placing them under the sun or moon, soaking them in sea salt, or smudging them with some kind of smoke like sage or sweetgrass. I have tried each and every one of these methods and gotten great results.
However, every now and then, I like to soak my crystals in the herb Mugwort. I use my tumbled stones for stone readings and over time, when they start to look dull in color and feel heavy, I pull out the Mugwort.
I fill a large pot with water and put a handful of Mugwort in it, bringing it to a boil. Once the water is boiling, turn the heat off and let the water cool. After it is completely cool, I place my crystals inside the water and let them soak. The soaking time can vary, anywhere between 30 minutes to a few hours. Use your intuition. Once they are done soaking, rinse them off with filtered water and place them in the sun for 1 hour.
This cleansing process always restores my crystals. They feel lighter and their vivid colors come back to life.