Дварим 27:6 Галаха: Mishneh Torah, The Chosen Temple, Sefer HaMitzvot & Sefer HaChinukh

Any stone that was chipped enough to catch the fingernail [to the same exacting standards] as with a shechita knife, in unfit for the ramp and for the altar as it says "With whole stones build the altar of God." Where did they get the stones? From virgin ground. They would dig down to where it was obvious that this soil is not for cultivating or building and take out whole stones, or they would take rocks from the Mediterranean Sea and build with those. Similarly, the stones of the Temple and the Courtyards were also whole.

Stones of the Sanctuary or the courtyard which were chipped or scraped cannot be repaired, rather they must be hid away. Any stone which was touched by iron may not be used for the Altar or the ramp because it says "for you have lifted your sword against it and desecrated it." One who built the ramp or the altar with stones touched by iron is flogged because it says (as a prohibition) "do not build them of hewn stones" but if one built with a chipped stone he has (only) transgressed a positive commandment.

An altar that [has a piece] broken off of its structure is valid as long as the missing piece is less than a hand-breadth and so long as what is left does not contain a chipped stone.