There are times when making a mosaic that you may choose not to grout a section of it. Some materials can't be grouted or look better not grouted. Porous materials in particular can't be grouted as they will take up the colour of the grout. This includes unsealed ceramic tiles and natural pebbles.
To protect your materials you can use the following technique:
Carefully cover the materials you wish to protect with masking tape or blue tack. I use masking tape, cut to size, and gently pressed onto the material.
Apply the grout to your mosaic, being extra careful on the areas you have covered
Allow the grout to dry for about 30 minutes and gently remove the making tape or blue tack.
If the area need a quick clean up, get a sharp tool and gently neaten up the area.