small quality factor
Quality factor when applied to a crystal resonator refers to the ratio of energy shuttled between Ls and Cs, to the energy dissipated by Rs.
These three components (Ls, Cs, Rs) are electrical representations to the mechanical motion of the crystal. Their values can be found by making electrical measurements at the crystal's two terminals. This measurement is done at the crystal's series-resonant frequency: \$ f_{series} = {1\over{2\pi\sqrt{L_s C_s}}}\$.

The component Cp is an actual capacitance between the two plates of the crystal. It is generally measured at a frequency far removed from the crystal's resonant frequency (usually at a much lower frequency).

Quality factor Q = \$ {2\pi f_{series} L_s}/R_s \$.