They absorb energy, redirect energy and give energy. Therefor cleansing skulls and crystals regularly is necessary as they will also absorb negative energy. It is likely that your crystals may absorb vibrations not only from you, but from anything in the environment where they are located.
To create positive energy within your crystals, you need to cleanse them frequently.
This is important as their use may be effected by negative vibrations within the stones.
There are several ways to clean your crystals. How different they may seem, there is one very important similarity: intention. Personally I prefer to use running water (if the crystal can have it) to clean my crystals and I am not a great fan of smudging. But I know some people are a fan of smudging and have great results with it. It is not the way you clean crystals, but the intention you have. If you can 'beam' your cleaning thoughts to the crystal you are half way. If you can use one of the methods I will mention to strengthen these thoughts, you are all the way ;).
I ask Saint Germain, one of the light-masters, to use his violet flame to clean the crystals while performing the cleaning ritual and imagine a violet light burning out all negative energy. And that works great for me.
Sound is one of the ways to enhance your intention. You can use singing bowls, shamanistic drums or Tibetan bells. Try to concentrate of the frequency or the drum rhythm. If you have a singing bowl, try to put a crystal in it when you let him resonate. In some cases it will sound like the crystal is singing along. Beautiful!
The most used smudge is white sage. You can by sage as leaves, smudge sticks, mixed oir stand-alone. It smells fantastic and is the smoke can be used to visualize it taking the negative energy away. You can also use incense sticks for cleaning. Some people use a combination of a shell (water), feather (air) and smoke (fire) to clean their crystals. This way al three elements with their own cleaning power can do their work.
Running water just out of the tap, or even better from a rippling creek, is a great cleaning method. Always be careful with water, for some crystals (like Selenite) can we ruined when using water. When using water imagine the water taking the negative energy along. For more power try to give the fresh water white light to fill the crystal with positive energy.
White Light
A very effective way to clean a crystal is to use white light. This can be combined with almost every other cleansing method. How does it work? Close your eyes and ask the angels, your spirit or the universe (depending on what you strongly believe in) to fill your body with white light. You will feel te energy. Imagine the white light coming out of your hands. Open your eyes and let the energy go from your hands to the crystal. Ask your guide to replace all negative energy with the positive energy from the white light.
The earth
Using the power of mother earth is also a way to clean your crystals. How? Easy! Just bury your stone in the earth and let the earth absorb the negative energy for about a night. Be sure your crystal is one that can take water, because the earth seldom is dry. And of course, be sure to mark the spot so you can find your crystals again.
Moon- and sunlight
The power of the sun and the full moon are easy ways to burn out negative energy. Sun or moon bathing not only clean, but also re-energize crystals. When it is possible leave the crystals in the moon for the night. You will notice they will shine as never before.
Be careful with putting your crystals in the sun. Some crystals will loose color. Amethyst is one of the crystals that can fade in the sunlight.
Use crystals to clean crystals. Selenite, amethyst and clear crystal will absorb negative intentions from other stones. Selenite is also self cleansing (just as kyanite is). Put your stone on top of a selenite, amethyst or clear crystal cluster and let the stone cleans itself en re-charge. Very effective are geodes, for this way the stone will be surrounded.
Without doubt there are other methods and - as I told earlier - they can be as effective as the ones mentioned above. Just be sure the intention is true, right and strong.