What factors can prevent crystals from forming with all faces visible

"All written work appears to the right of the red margin line a crystal are perpendicular to the crystal faces and intersect in the center of the crystal. ... 28. What factors in nature can prevent natural crystals from forming with all faces visible? Eventually, different layers, or zones, of soil become noticeable" (www.lcps.org/cms/lib4/VA01000195/.../2014%20semester%202%20Booklet.pdf)

Hope this helps

the relative atomic mass of an element is the mass of one atom of that element, relative to 1/12th of the mass of a  1212c atom. to convert to an absolute mass in kg, it is necessary to multiply by  1.661×10−271.661×10−27.

the relative atomic mass is roughly equal to the number of neutrons, plus the number of protons, in the nucleus of the atom.

for elements with more than one stable isotope (different types of nucleus with different numbers of neutrons but the same number of protons), the quoted mass is usually the average mass, weighted by the relative abundance of the different isotopes.