“…A stone that is chipped to the extent that it can be sensed by a fingernail… is disqualified for use in the altar or the ramp, as [Devarim 27:6] states: “You shall build the Altar of G-d with whole stones”… If a stone was damaged or touched by iron after it was built into the Altar or the ramp, that stone is invalidated…” – Rambam, Hilchos Beis Habechirah 1:14-16

כל אבן שנפגמה כדי שתחגור בה הציפורן . . הרי זו פסולה לכבש ולמזבח שנאמר אבנים שלמות תבנה את מזבח ה’ . . אבן שנפגמה או שנגע בה ברזל אחר שנבנית במזבח או בכבש אותה האבן פסולה – רמב”ם, הל’ בית הבחירה פ”א הל’ יד-טז

The Rambam writes that a stone that is chipped to the extent that a fingernail can sense the damage to its surface may not be used in building the mizbeiach, the Altar. Regarding a stone already built into the mizbeiach, however, the Rambam rules categorically, “if a stone was damaged… after it was been built into the Altar or the ramp it is invalidated.” He does not state the amount of damage that invalidates it. This implies that once a stone has become part of the mizbeiach the damage that disqualifies it is not defined by an amount or measure; any damage to the stone ruins it.

The reasoning for this is because the prohibition of using damaged stones when constructing the mizbeiach is learned from the verse, “You shall build the Altar of G-d with whole stones” (Devarim 27:6). Hence, a stone that is deficient and is no longer “whole” is disqualified for use in the mizbeiach’s construction. “Deficiency,” however, is determined by an actual amount of damage—enough damage that a fingernail can sense it.

Conversely, a stone’s disqualification if damaged after becoming a part of the mizbeiach is due to another factor: its status as “a damaged segment” of the mizbeiach. Now, the prohibition to damage the mizbeiach is taught in the verse that contrasts our treatment of the mizbeiach with our obligation to destroy altars used for idolatry: “You shall destroy their altars… Do not do so to Hashem your G-d” (Devarim 12:3-4). Accordingly, the definition of destruction or damage with regard to the mizbeiach is equivalentto the definition of destruction as it pertains to our obligation to abolish idolatry.

Therefore,just as we are obligated to abolish idolatry entirely and not to maintain anything that is consecrated to it (see Devarim 13:8), likewise, any damage whatsoever to a stone that is already part of the mizbeiach disqualifies it, even if that amount of damage does not “constitute” deficiency. The Rambam therefore states that a stone that was damaged after it was built into the mizbeaich is invalid, regardless of the amount of damage.

—Likutei Sichos, vol. 29, pp. 153-158