You're describing the grain size of a rock. A grain size of 0.05 cm falls within the sand category.

Here's a breakdown of the common rock classifications based on grain size:

* Clay: < 0.0039 cm

* Silt: 0.0039 cm - 0.05 cm

* Sand: 0.05 cm - 2 mm

* Gravel: 2 mm - 7.6 cm

* Cobbles: 7.6 cm - 25.6 cm

* Boulders: > 25.6 cm

Keep in mind that these are general guidelines, and there can be some variation depending on the specific rock type.