The diabolo is composed of two hollow domes and an axle, these days everything is made of plastic or rubber except for the axle that has to be in steel. A good Diabolo must have a wide spline in order to let you perform nicer and harder figures.
the sticks are usually made of pinewood or beech-wood and the string has to be weaved in cotton and nylon for better resistance. For a good trick execution the string has to be thin.