Waterjets can cut mostly about any material that can be made into a sheet and placed in front of them.
The most popular materials are stone,ceramic and glass,because the waterjet can get intricate shapes not possible using traditional machining methods. These materials are popular with artists who like to work with these materials and waterjets because it lets them create almost anything they can envision.

Waterjets also commonly cut metals (especially aluminum, because it's relatively soft and cuts quickly), because waterjets can cut intricate shapes to a high precision quickly and economically. Since metals are the most common material cut by machining shops, waterjets tend to cut a lot of metal.

Among the very few materials that waterjets cannot cut are diamonds and tempered glass. Diamonds are too hard to cut (and there may be a few other very hard materials that can't be cut). Tempered glass will shatter when it is cut with a waterjet (tempered glass is designed to shatter when it's disturbed and is frequently used in windshields for this very reason).