We analyze the influence of the dielectric constant of the slab on the quality factor (Q) in slab photonic crystal cavities with a minimized vertical losses model. The higher value of Q in high-epsilon cavity is attributed to the lower mode frequency. The Q ratio in a high-epsilon (silicon) vs. low-epsilon (diamond) slab is examined as a function of mode volume (V(m)). The mode volume compensation technique is discussed. Finally, diamond cavity design is addressed. The analytical results are compared to 3D FDTD calculations. In a double heterostructure design, a Q approximately 2.6 x 10(5) is obtained. The highest Q approximately 1.3 x 10(6) with V(m)=1.77 x (lambda/n)(3) in a local width modulation design is derived.