Crystals need to be cleaned, when you buy them and after wearing or using for healing. Always cleanse jewelry that comes to you from someone else, as it can hold their negative vibrations and pass them on to you.
Crystals are not friable or jointed can be held under running water or immersed in the sea or salt water. As you do so, hold the intention that all negativity will be washed away and the crystal reenergized.
A few crystals never need cleansing. Citrine, Kyanite and Azeztulite are self-cleaning. Clear Quartz and Carnelian cleanse other crystals, and are especially useful for delicate and friable stones, but may need cleaning themselves afterwards.