The most desirable characteristic for constraining the relative age of rocks is cross-cutting relationships. This principle, also known as the "principle of superposition," states that any geologic feature that cuts across another feature must be younger than the feature it cuts.

Here's why cross-cutting relationships are so powerful:

* Direct Observation: It's a direct observation of geological events. We can see the intrusion of a younger igneous rock into older sedimentary rocks, or a fault cutting through existing layers.

* Clear Temporal Relationship: It establishes a clear temporal relationship between the features. The feature doing the cutting (e.g., the igneous intrusion) must have formed after the feature being cut (e.g., the sedimentary layers).

* Universally Applicable: This principle is universally applicable to any type of rock and geological feature, making it a fundamental tool in relative dating.

While other characteristics like fossil content and rock types can be helpful in determining relative age, they often require additional interpretation and can be subject to more ambiguity. Cross-cutting relationships offer a more direct and reliable method for establishing the relative age of rocks.