Common materials show a limited range of refractive indices over the visible and infrared wavelengths, and their values are not easily tunable once the material is chosen. Here, self-assembled metal nanoparticle arrays are proposed as an effective optical material with a large range of possible refractive indices that are nearly dispersionless over broad wavelength ranges. The material can be potentially fabricated over a large curved surface, and the resulting index is even more tunable by various postprocessing methods, such as material substitution or mechanical stretching. We achieve the highest refractive index of 5.0 at resonance in the visible, exceeding 4.2 over a broadband wavelength regime out of resonance. The key differences from previous studies of self-assembled metal particle arrays are the consideration of the diamagnetic effect and the careful choice of the ligands, which determine the gaps between particles. The sensitivity of the effective index to the size and material of the gap regio...