1. A method for determining variations among multiple three-dimensional stone images extracorporeally, which involves during a lithotripsy process performed on a human body, multiple three-dimensional stone images taken from a stone location in the human body are processed outside the human body using the following steps: using a processing unit to execute a computing process for the multiple three-dimensional stone images one by one in an order from the first taken said image to the last taken said image, wherein the computing process involves using an entropy texture equation or an inverse difference moment (IDM) texture equation to calculate a texture feature value for each said three-dimensional stone image, in which the entropy texture equation is defined as E=−Σ i,j P(i,j)log P(i,j), and the inverse difference moment (IDM) texture equation is defined as IDM = ∑ i , j ⁢ P ⁡ ( i , j ) 1 + ( i - j ) 2 , where P ⁡ ( i , j ) = C ⁡ ( i , j ) ∑ i , j ⁢ C ⁡ ( i , j ) , and i, j are image grayscale pixel values, and C(i, j) is a number of appearances of (i, j) whenever a combination of a pitch d and an angle θ between pixels exists; and when the texture feature values calculated using the entropy texture equation or the inverse difference moment (IDM) texture equation substantively stop changing, making the processing unit output a prompt signal.