  One of the key differences between normal neutron and (bare) quark stars is
relevant to the fact that the former are gravitationally bound while the latter
self-confined unless their masses approach the maximum mass. This difference
results in the possibility that quark stars could be very low massive whereas
neutron stars cannot. Mountains could also be build on quark stars if realistic
cold quark matter is in a solid state, and an alternative estimation of the
mountain building is present. As spinning compact objects with non-axisymmetric
mass distribution will radiate gravitational waves, the equations of states of
pulsars could be constraint by the amplitude of gravitational waves being
dependent on the heights of mountains. We then estimate the maximum mountains
and thus quadrupole moment on solid quark stars, to be consistent with that by
Owen (2005) if the breaking strain is 0.1, addressing that a solid quark star
with mass < 10^{-2} Msun could be `potato-like'. We estimate the gravitational
wave amplitude of solid quark stars with realistic mountains to be order of h0
~ 10^{-27}.
