Grout

A substance which fills the gaps (joints or interstices) between tesserae in mosaics. One purpose of grout is to strengthen the work and make it longer-lasting by keeping water out of the joints. It also has the effect of completing a design - ungrouted mosaic joints can be distracting to the eye. Most often a neutral colour is chosen for the grout, but vivid colours can be produced by adding acrylic paint or commercial cement dye. There are two common types of grout. One is ready mixed, and is also used as a tiling adhesive. The other is cement-based and is a dry powder that is mixed with water when needed. Here's an example

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