If you've purchased a tile that doesn't have matching trim pieces, and you don't want to use a metal trim, you'll need to be a little creative in order to find something to use for the sides/top/bottom of your niche.

You could look at dolomite curb/ledge pieces (or if your niche allows - you could use dolomite tiles which will allow you to have thinner edges around your nicheI . Dolomite is a natural stone (related to marble) that will have some white/gray/tan in it.

You should use a solid piece for the bottom of your ledge (and will need another solid one if your niche will have a shelf). If your niche isn't wide, tile will work for both. You will only see the edge of the dolomite tile - if you hand select your pieces, you can find ones that will work with the colors in your faux marble tile.

Both of the photos below are dolomite tile:

You could also line your niche with Thassos - it is white with a more visible crystalline structure. There are different grades of Thassos tile -some are very white while others have a softer white look to them.

You could then use dolomite or thassos for the top of your curb. It's much better to use a solid piece of stone (or other material) on the top of your curb.

What are you using for your countertop? Sometimes people will have niche pieces/curbs/pony wall caps/etc. fabricated out of their countertop material. If you select a solid white countertop material, you could have that fabricated to work with your shower tiles.

Below is a real marble shower with mother of pearl mosaic tiles on the back of the niche. The top/sides/bottom of the niche looks like it has been made with white quartz. I've seen this done many times - often a solid white countertop will be used on the vanity. Some big box stores sell solid white shower curb pieces that can be used to frame niches.

Below is another niche that appears to be framed with quartz (also, a solid piece of quartz was used for the back of the niche):

The shower below has Bedrosian Cloe white tile on the walls with a chevron marble mosaic on the back. Solid white material has again been selected to frame out the niche. They could have used matching Cloe White pencil trim but decided to use the solid white material instead.

The below photo shows a shower that has a real marble mosaic used on the walls + on the back of the niche. The niche has been framed with a faux marble quartz (I assume they used this quartz for the curb + vanity countertop):

You could have a solid niche made out of Corian (or other material). You could select the right "white" color that would work with your tiles + have it custom sized:

This last photo shows some pencil trim that was used to frame out the niche. It appears to be made out of natural stone (marble or dolomite). I am not a big fan of pencil trim (as used below) - there are trim pieces that have squared off edges that I think look nicer than rounded pencil trim.

If the mosaic tile that you are using for the back of your niche has black in it, you could find black marble or porcelain trim pieces to finish the edges of your niche. However, I think that may look busy with faux marble tiles on the wall. You'd have to lay out pieces in order to see how it all works together.

Make sure that your tile pro sizes your niche so that everything lines up nicely with your wall tile. See how the top/shelf/bottom of the niche shown below all line up with a grout line on the wall? Looks much nicer than if there are little slivers of tile above/below the niche. They also matched the width of the niche to the width of the tile in order to have either a whole piece of tile on each side of the niche or a half piece - again, no slivers of tile.

The numbers above are just a reference for where your niche should be located. Also, it's nice to add a shaving niche down low to put your foot while shaving your legs (you can also add a shaving bar that is installed on the wall instead of a small niche).