Category: solids...

solids are generally crystalline (have a high degree of order in atomic arrangement) or non-crystalline (amorphous of quasicrystalline)
becoming solid and stable like an oak tree, not blown around by a storm of change
a solid forms from a liquid or gas because the energy of atoms decreases when the atoms take up a relatively ordered three-dimensional structure
all solids have, for example, the ability to resist forces applied either perpendicular or parallel to a surface
I have found that hollow, which even I had relied on for solid. (HD Thoreau)
differences in size, density, solubility, and magnetic properties separate one solid from another; filtration separates an insoluble solid from a liquid
in panning for gold, gold is separated from gravel as gold is heavier and sinks more quickly in the bottom of the pan