Why not use a piece of solid black granite or quartz? I am sure you could find a remnant at your local stone yard that would work. I don't like the option you have posted....it clashes with your tile flooring.

I'd be a LOT more concerned with the waterproof integrity below the CRACK, which has now had a water seep through it for how long? I;d be very concerned as ot how this was originally built and waterproofed.

" We used BEVELED PIECES .( read first mistake, and no over hang)

I smell trouble beneath.

Was you original marble Carrara out of Italy? I say this because from the looks it may be another stone; look at other similar marbles if you want to get a bit closer to the materials installed.

That's low quality "chinese carrara" with a whole lot of iron in it, which has rusted. And it is set over a very suspect substrate. You have far bigger issues than you think.

A coordinating solid surface would outperform any estone or natural stone in this application.

I’d bet the curb is wooden 2x4’s, with the pan membrane screwed through into them. Slow leak, high rot, plus wood expansion equals self destruction of the curb, and the shower leaking you just havent found.

^^

I will echo that unfortunate sentiment.

You definitely need to see what the curb looks like once the beveled tiles have been removed. Unfortunately, there are probably water damage issues as has been suggested in several other comments. Curbs are one of the first places where water intrusion occurs causing damage (bench shower seats also often have issues).

IF your contractor confirms that there is no water damage (seems unlikely) - I would install either a solid surface or quartz curb in solid black (or a solid white that matches the background of your installed marble). I would NOT use a faux marble quartz curb - faux marble never looks real - and should never be installed directly next to real marble. You also could try to find a black granite curb that works with your tile.

I would not install the marble curb that you have in the photo - the background of the marble is much grayer than your marble floor tiles.

FYI - the existing tiles installed on top of your curb do not look like Italian Carrara marble. There aren't really any of the typical veining/spots that are in Italian Carrara.

take the glass out and pull all the tile off the curb now. You really should not be using the shower again until you know the condition of the curb, and if water has gotten under the floor or the shower pan.

If it all has to come out, there is only one word I can suggest: Shluter.

White or black-not gray! Good luck.

Look for Thassos Marble Shower Curb.

We used a honed piece of soapstone (which was also used for our kitchen countertops and the bathroom vanity counter) and it looks terrific with the honed marble tile.

We used the engineered marble threshold from Home Depot. It looks good and has held up great.