Exodus 20:20

וְאִם־מִזְבַּ֤ח אֲבָנִים֙ תַּֽעֲשֶׂה־לִּ֔י לֹֽא־תִבְנֶ֥ה אֶתְהֶ֖ן גָּזִ֑ית כִּ֧י חַרְבְּךָ֛ הֵנַ֥פְתָּ עָלֶ֖יהָ וַתְּחַֽלְלֶֽהָ׃

And if you make for Me an altar of stones, do not build it of hewn stones; for by wielding your tool upon them you have profaned them.

You have to admire the intensity an animal sacrifice: the emotion of the moment the owner lays their hands on the head of the animal, the Kohen gathering the blood and dashing it on the altar, the cloud of pungent smoke rising upward. Each stage of the sacrifice must have felt momentous. Ever since the destruction of the Temple, prayer has been the substitute for these sacrifices. Instead of a morning sacrifice, we have a morning prayer service. Instead of smoke from the altar, our prayers ascend in its place, and instead of an altar, there we stand. 

When it came to animal sacrifice, the altar itself lent meaning to the moment. It had to be made of dirt and rocks, and the rocks used for the altar could not be cut or hewn. Whatever shape they naturally were was how they must remain.We learn from the altar what it takes to be ready to pray. No heirs need be put on, no hiding of who one is in that moment will make us a more fitting vessel to offer a prayer.  If one wanted to offer a sacrifice, the first step was to prepare an altar, and if a stone in the altar was hewn, taking away from its naturally imperfect shape, it could not be used. If we want to pray, the first step is not preparing an altar, but preparing ourselves. The imperfection of the unhewn stones a reminder that we need not pretend to be any more or less than who we are in that moment. Rather it is because of out imperfections that our prayer is heard.