Leaves of the majority of plants contain calcium oxalate (CaOx) crystals or druses which often occur in spectacular distribution patterns. Numerous studies on CaOx in plant tissues across many different plant groups have been published, since it can be visualised readily under a light microscope (LM). However, there is surprisingly limited knowledge on the actual, precise distribution of CaOx in the leaves of quite ordinary plants such as common native and exotic trees. Traditional sample preparation for the documentation of the distribution of CaOx crystals in a given sample – including overall distribution – requires time‐consuming clearing procedures. Here we present a refined fast preparation method to visualise the overall CaOx complement in a sample: The plant material is ashed and the ash viewed under the polarising microscope. This is a rapid method which overcomes many shortcomings of other methods and permits the visualisation of the entire CaOx content in most leaf samples. Pros and cons in comparison with the conventional clearing technique are discussed. Further aspects for CaOx investigations by micro‐CT and scanning electron microscopy are discussed. LAY DESCRIPTION: Striking patterns of calcium oxalate (CaOx) crystals and druses (crystal aggregates) can be found in leaves of higher plants such as common trees and shrubs. They are easily visible under a light microscope, but this requires a chemical clearing procedure to render the samples transparent for polarising microscopy. Here we present a refined fast preparation method to visualise the overall CaOx complement in a sample: The plant material is incinerated and the ash viewed under the polarising microscope after immersion in oil. This rapid method overcomes the shortcomings of other methods; burning eliminates all organic components such as cellulose, which would cause a disturbing background signal, and oil immersion provides good transparency of the samples. Thus, the burnt CaOx crystals are clearly visible whereas other ash components appear invisible in the polarising microscope. Pros and cons in comparison with the conventional clearing technique are discussed. With this simple preparation method, CaOx distribution patterns in leaves may not only become favourable motives for amateur microscopists; for botanists they are a key for the understanding of their functions which are still in parts enigmatic and a matter of speculations. [ABSTRACT FROM AUTHOR]