Genuine marble tiles are flat and square-cut--as opposed to ceramic and many other styles that tend to be curved--because the marble tile isn't formed in molds but cut right out of the natural stone. Marble tiles are also prone to scratching from sand or other abrasives. All of this has implications for the spacing-and-grouting process. To avoid scratching the tile, use grout with no sand in it, which necessitates tight, consistent spaces on your grout line. This will create a smooth, slab-like surface out of the flat tops of the marble tile.
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Spacing the Tiles
The spaces between your tiles should be no more than 1/8-inch wide (a 1/16-inch space is better, if the edges of your tiles are straight enough to accommodate it). Always use plastic spacers to get that consistent line. Because the line is so thin, do not use the style of spacer that drops into the space and stays under the grout, but use removable spacers. Four-way spacers that you set at the corners are best. After you press your tile to the adhesive, keep the spacers in place as you level out the tiles with the edge of a straight board, making sure they're even across the spaces. After you lay all of the tiles, pull the spacers out and let the tiles set overnight.
Grouting the Lines
"Unsanded'' grout (which simply means no sand is added to the mortar) can only be used in thin grout lines, as it will crumble out of wider ones. Unsanded grout is thinner than standard grout and will fill entirely between your thinly spaced tiles without danger of obstructions or air pockets. Because it contains no sand, it will not scratch up the tiles as you apply it. Press it thoroughly into the lines and wipe off the excess with a damp sponge.
Sealing the Lines
Sealing very thin grout lines is a challenge, because you don't want the sealer to dry on the marble. Don't use the standard grout sealer with the sponge applicator, as this will be too wide for your 1/16- or 1/8-inch lines. Use a very thin artist's brush and carefully brush the sealer right on the line. Have a dampened paper towel handy to wipe off any sealer that gets on the marble face.