Normally, the stones used for this therapy are basalt, given their ability to keep heat. Besides, they must be extremely smooth to avoid irritating the skin. The number of stones a therapist should have can vary between 20 and 30. Yet, some professional massage therapists usually have more than 45 or 60. At least, they should have two oval stones approximately 20 cm (8 inches) long by 15 cm (6 inches) wide, seven that fit in the palm of his hand, and eight small ones about the size of an egg.