Also finished off my mosaic coaster today, by gluing some felt on the back.
Not sure I'll bother doing mosaics again, not really my thing. But if I did I wouldn't just stick the cut tiles with PVA as it really needed some sort of thicker glue to enable you to make the pieces level. My grout says it is the adhesive too, so that would have been the thing to use really. Forgot to mention the central tile got turned somehow on its way home but I preferred it that way and left it diagonal.