For venetian glass, use very spotty to moderately spot-dyed fabric hooked in sequence. To simulate pebbles, use moderately spotted or flat dyed wool and hook in circles. Hook the tesserae in opposite directions occasionally. Be careful not to pack your loops unless you want the texture of little bumps. If the back of your hooked item will be covered, such as for a cushion or stool cover, it's fine to cross the hooked grout lines with the tesserae wool.
Crossing makes it easier to hook a spot dye in sequence.
However, on a floor rug each tessera must be completed separately because crossovers on the back will wear more quickly. Sometimes it is desirable to hook the grout last for special effects. For a very fine line, hook the tesserae first, then put the grout in with yarn, making the loops double the height of the tesserae. When you've finished hooking the grout, cut the yarn loops to the height of the tesserae. Once you've tackled a geometric, you can move on to more creative patterns.