Focusing of ultrasound waves is critical to a number of clinical and industrial applications including biomedical and underwater imaging, nondestructive evaluation and material processing. This paper discusses the use of a novel 'add-on' gradient refractive index (GRIN) metamaterial structure made of concentric shells, to focus ultrasonic waves generated by conventional transducers. Analysis based on the Huygen's principle and numerical simulations is used to design the geometric and material properties of the proposed structure, whose working is demonstrated through experiments. Varying the shell material or thickness is shown to offer an elegant and straightforward way to tailor the focal spot inside the target material. The concentric-shell GRIN lens proposed here has a simple design, and has a potential to be used in dynamic focusing without advanced lenses or electronic steering. © 2021 Elsevier B.V.