The property of a rock that remains unchanged by mechanical weathering is its chemical composition.

Here's why:

* Mechanical weathering breaks down rocks physically into smaller pieces. Think of it like smashing a rock into pebbles. The chemical makeup of the rock itself doesn't change.

* Chemical weathering, on the other hand, alters the chemical composition of the rock. Examples include dissolving limestone with acid rain or oxidation (rusting) of iron-rich minerals.

Let me know if you'd like to learn more about mechanical or chemical weathering!