You wouldn't need a specific tool to observe the luster of a rock. Luster is simply how light reflects off the surface of a rock, and you can observe it with your own eyes.

However, to make the observation easier and more accurate, you can use these tools:

* A hand lens: Magnifying the surface of the rock can help you see the details of how light reflects.

* A strong light source: Shining a bright light onto the rock will make the luster more apparent.

* A dark background: This helps to contrast the rock and make the reflection more noticeable.

Here are some additional tips for observing luster:

* Look for the overall appearance of the reflection: Is it metallic, glassy, pearly, dull, or earthy?

* Pay attention to the direction of the light source: How does the reflection change depending on the angle of the light?

* Compare the luster to known examples: If you're unsure about a particular luster, you can compare it to examples in a field guide or online.

By using these techniques, you can more easily and accurately observe the luster of a rock.