The most widespread classification system for pebble shapes in geology is the Zingg system which relies on several length measurements. Here we propose a completely different classification system which involves counting static equilibria. We show that our system is practically applicable: simple hand experiments are suitable and easy to use to determine equilibrium classes. We also propose a simplified classification scheme called E-classification which is considerably faster in practice than the classical Zingg method. Based on statistical results of 1000 pebbles from several different geologic locations we show that E-classes are closely related to the geometric shape of pebbles. We compared E-classes to the Zingg classes, and we found that all the information contained in Zingg classification can be extracted from equilibrium classification. However, the new method is more sophisticated: it may help to identify shape attributes not discovered so far and it is able to store information on special geometries, e.g. on crystal shapes.