We show that it is possible to prepare and identify ultra-thin sheets of graphene on crystalline substrates such as SrTiO₃, TiO₂, Al₂O₃ and CaF₂ by standard techniques (mechanical exfoliation, optical and atomic force microscopy). On the substrates under consideration we find a similar distribution of single layer, bilayer and few-layer graphene and graphite flakes as with conventional SiO₂ substrates. The optical contrast C of a single graphene layer on any of those substrates is determined by calculating the optical properties of a two-dimensional metallic sheet on the surface of a dielectric, which yields values between C = −1.5% (G/TiO₂) and C = −8.8% (G/CaF₂). This contrast is in reasonable agreement with experimental data and is sufficient to make identification by an optical microscope possible. The graphene layers cover the crystalline substrate in a carpet-like mode and the height of single layer graphene on any of the crystalline substrates as determined by atomic force microscopy is d_SLG = 0.34 nm and thus much smaller than on SiO₂.