Curable ink compositions include a curable aromatic monomer composition, and surface treated metal oxide nanoparticles. The surface treated metal oxide nanoparticles are surface treated with a mixture of at least two silane-functional surface treatment agents, at least one aromatic-containing silane-functional surface treatment agent and at least one silane-functional surface treatment agent comprising a co-polymerizable group. The curable ink composition is inkjet printable, having a viscosity of 30 centipoise or less at a temperature of from room temperature to 60°C, and is free from solvents. The curable ink composition, when printed and cured, has a refractive index of 1.55 or greater, and is optically clear.