  It is conjectured that cold quark matter with much high baryon density could
be in a solid state, and strange stars with low temperatures should thus be
solid stars. The speculation could be close to the truth if no peculiar
polarization of thermal X-ray emission (in, e.g., RXJ1856), or no gravitational
wave in post-glitch phases, is detected in future advanced facilities, or if
spin frequencies beyond the critical ones limited by r-mode instability are
discovered. The shear modulus of solid quark matter could be ~ 10^{32} erg/cm^3
if the kHz QPOs observed are relevant to the eigenvalues of the center star
oscillations.
