Sand shading is a technique to add depth and realism to marquetry. As its name suggests, you achieve this by immersing the piece in hot sand.
When deciding where to sand shade, consider the light source for your motif. The direction of light incident on the piece is key! An object beneath another will be darkened as will objects that curve away from the source. It’s an exercise in three-dimensional thinking.
In the drawing below, I use a series of “x”s to denote where shading should occur. When executing this design the temperature of the sand, its topography, and length of time the piece is exposed to it govern the effects. This differs in each instance. So, it is important to carefully monitor the process!