These flat black buttons measure 18mm diameter and are 4mm thick, so will sit happily alongside all your other tiles. The tiles are opaque glass and are a lovely glossy black. The photo makes them look a bit dull as they are slightly dusty - sorry, I haven’t had time to wash them. Rest assured that they will come up beautifully when you get round to doing it! The tiles are supplied in 250g bags, which contain around 110 pieces. The bottoms are completely flat with a few indentations to help your adhesive stick.