The Vellum Grains shelf tool lets you pick a piece of geometry and configure it as Vellum grains. It will then be added to the active simulation and merged with any other existing vellum objects.

The object will be filled with points and solved with the points treated as hard spheres, giving a dry-sand effect.

This is very similar to the Dry Sand tool on the Grains shelf. However, it works inside the Vellum world, so you can have the grains interact with other Vellum objects.

One important thing to note is that grains require higher Substeps on the Vellum Solver other Vellum objects. This is not an issue when using the shelf tool, but if you are creating the network manually, you will need to increase this value to at least 5 to prevent the grains from exploding. There is also a special Grain Collisions section on the Advanced tab of the Vellum Solver for more control.

Unlike the other vellum objects, grains has its own Vellum Configure Grain SOP node to set constraints, because it works so differently. There are two ways to create grains. You can either take the incoming points and add grain to them, which will cause them to start bouncing with each other. You can alternatively Create Points from Volume, which is the default. This will treat the incoming geometry as a closed volume, fill it with points, and then configure them as grains.

Video credit: Moeen Sayed