A new crystal should always be cleansed before you work with it for healing.
This is not actually to clean dirt from it, but the unwanted energy it will have gathered from other people on it's journey to you.
You also need to cleanse them reguarly once you start working with them, as they will draw the negative energies into themselves and if you don't cleanse them they will stop working and if taken to the extreme they can actually shatter.
There are many ways suggested for cleansing, but you should take care before you decide on which method to use. For instance, some crystals would be destroyed by water, while others would fade in sunlight, and burying one in the garden might be fun for your dog but a bit of a nightmare when you're trying to find them again!
I recommend 2 methods, which are safe for any crystal, even those set in delicate jewellery. They are a great way to start until you get to know your crystal and very effective and gentle. I still use them myself for cleansing my crystals.
An Amethyst bed or druze
(this is a piece of amethyst with a number of natural points, it can be flat or curved) makes an excellent cleansing tool. Just place your crystals gently onto the amethyst and leave them for 3 to 4 hours.
Amethyst is a very balancing crystal and great for cleansing negative energy from your energy field, a larger piece is wonderful for balancing the energy in a room and it will gently remove the negative energy from the other crystals you have been using.
The natural points will direct energy away, which is why it's important to work with a natural bed or cluster with a number of natural points that will direct the negative energy away from your crystals. A polished piece of amethyst isn't suitable as a cleansing tool.
Take care when you're placing the crystals onto the amethyst points, don't push them or scrape them across the points, amethyst is a harder crystal than many others can could scratch your softer crystals if you're too rough.
(you can get it from the supermarket) is also a very good way of cleansing the negative energy from your crystals.
Pour the dry seasalt into a clear glass bowl and sit a smaller clear glass bowl into the salt so that it is surrounded by the dry salt - a bit like melting chocolate in a bowl of hot water.
It's important to use clear glass, if you use coloured glass you will be introducing colour energies, and glass allows the energies to pass through without adding any energy of its own.
Put your crystals into the smaller bowl (just dry, there's no water involved) they will cleanse through the glass. Again, leave them for 3 or 4 hours, that is plenty of time to cleanse the negative energy from them and rebalance them ready for working with again.
If you're comfortable with dowsing with a pendulum you can cleanse your crystals in this way as well. Just ask your pendulum to remove the negative energy from your crystals and then hold it over the crystals and let it move as it wants until it comes naturally to a halt. This will make sense if you are an experienced dowser - if you've never worked with a pendulum - it'll make no sense at all!
When to cleanse
As well as cleansing a crystal when you first get it home, you will need to keep the erngy balanced as you work with them.
As you work with your crystals they draw negative energy from you into themselves and the balance of their energy is changed, this is why they need to be rebalanced.
You should cleanse your crystals when you 'feel' that they need it. It will feel as though their energy is running down and they are no longer as effective as they were.
As you get to know them, you will be able to tell when they need cleansing - but to start with, about once a month is a good general guide.
There is no hard and fast rule to follow. Some crystals can work for a long time between cleansing, there are people that say citrine and amethyst never need cleansing - but I wouldn't like to treat them like that and cleanse my citrine and amethyst at least every six months.
There are other crystals that require much more care. For instance, if you are working with malachite to ease pain, you should really cleanse them every day.