I am installing natural marble 12x12 tiles on the floor.
Do I have to use grout?
I am using ¼ “ cement board for the sub floor. I think there is only one layer of ¾” plywood.

Second. I want to install glass mosaic I am coloring and myself. I was told I have to use a minimum ½” cement board. I want to use ¼” since the cabinets are already installed and I don’t want to remove them. Can I use the thinner cement board?

Since I am making the colored glass for the mosaic there is no manufacturer instructions. Should I use Flexbond to attach the glass tile on the wall. They will be harlequin shape about about 2”x2” or so.

Hi, I was here looking for an answer to your grout question myself. I suspect it is an opinion on wheter or not to use grout but I am curios. I don't want to use it myself.
As for your question on the sub-floor, I am on a slab but I am using Hardiebacker Boards for my tile backing and here is a link where they say you can use 1/4" for subfloor.
James Hardie North America - 1-888-jhardie (http://www.jameshardie.com/backerboard/#1). I chose it over cement board because I felt that it was a more durable product. This link claims that it will do what 1/2 cement board will do on the floor.

Thanks Massterbath,

I already went to the hardi board site and it helped. I really wanted to know about the wall app. As far as thickness.

The floor app new question is if I am using marble on the floor do I have to use grout in between

I think the answer is, even if you are butting granite tile edge to edge you still need to use unsanded grout.

I am using 1/2" hardibacker for my walls. I am also wondering about the grout/no grout issue because I am using the same tile in my shower - walls. Will I have issues without grouting.