Object size inspection is an important task and has various applications in computer vision, for example, automatic control stone-breaking machines. In this paper, an algorithm is proposed for image segmentation on size inspection of almost round stones with strong textures or almost no textures. We use one camera and multiple light sources at difference positions to take one image when each of the light sources is on. Then we compute the image differences and threshold them to extract edges. We will explain, step by step, picture taking, edge extraction, noise removal, and edge gap filling. Experimental results will be presented. Through various experiments, we find our algorithm robust on various stones and under noise.