Would it be bad design to use small mosaic stone for countertops?

The biggest problems I see with using such small pieces to cover a countertop would be the amount of grouting such a job would involve and the fact that smaller tiles have a much higher tendency to pop up then larger tiles do. With 5/8" tiles, you'll have very little adhesive holding each tile to the countertop, so a lot of your structural strength will actually come from the grout itself adhering the mosaic tiles together. Setting mosaic tiles also takes quite a bit more time and patience than larger porcelain or ceramic tiles (4"x4" or larger).

I'm assuming, of course, that these mosaic tiles are not already on mesh backers. If this is the case, it would be quite a bit easier to install and the mesh backing will give more structural integrity by holding the tiles together, so even if a tile tries to come loose from your adhesive, the surrounding tiles will help hold it in place.

Another point you will want to consider is sealing, as a stone countertop will need to be sealed (the tile itself, not just the grout lines), whereas ceramic or porcelain tiles don't require sealing.

Aesthetically, I don't see why a countertop made up of mosaic tiles would be any less desirable than one made up or larger tiles. As with any home improvement decision, it's largely about personal taste. It probably wouldn't be quite as strong structurally, but otherwise it's basically an aesthetic choice.

The best idea in regard to wear and maintenance would be to continue with your plan to tile the counters with larger ceramic, porcelain, or granite tile. Select a tile that compliments the mosaic tile you already have and use the mosaics for a back splash. That would look great and save you a lot of cleaning.

Be sure to seal the grout with a grout sealer after the grout is dry and the installed tiles are polished clean of grout. That will also keep the maintenance more manageable.

Enjoy the project!

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