A jar of pebbles is a mixture because not all the pebbles are the same.
A mixture is by definition two or more components added together that can mix evenly or unevenly depending on different phases, which scientifically is referred to as homogeneous and heterogeneous mixtures respectively.
In the terms of the pebbles they are of different shapes and contain different chemical composition make-up.
When labeling a sample a mixture or solution, it is important to look at both the difference in physical state AND the difference in chemical composition.
-Physical state mixture: Glass of ice water A glass of ice water is a mixture because the ice is in a different physical state than the liquid water.
-Chemical composition mixture: Mixture of various gases Even though these are both gases, we call it a mixture because they have different chemical compositions.