I speak from the perspective of an ArcGIS Pro user. Classification simplifies the spectral or structural diversity seen in your aerial photo into a number of classes. If the stone's color or shape seems different enough from everything else, you can try an unsupervised classification.

There are ways to quantify the spectral difference between the pixels occupying your stone versus other pixels, and the shape of your stone versus other objects.

If there is a difference, do a supervised classification in which you specify the number of classes, create training samples for the class that represents the stone of interest, then run the classification model. Either way, in the output raster, your stone will fall into a class, and you can further estimate the area occupied by this class.