The only other possibility might be using a utility knife and carving the mortar out.

It's a personal choice. You will have to seal them. Personally I would never put marble in the shower, but several people do. Search marble showers on here & read what people say. There have been tons of discussions about it.

Dottie - I could not have done this without an educated designer - it's not just about the design choices although I would never have come up with design choices as my brain isn't hardwired in those kinds of visually imaginative ways. I can appreciate things but I don't know how to pull them together and I wouldn't have the ability to figure out where to even start in terms of the sea of vendors. I am good about researching appliances and functions but get lost in the sea of visual possibilities :-) And I can't even imagine dealing with codes and such since Los Angeles is code crazy - I wanted to move some floor electrical outlets up a bit from the floor as I found it difficult to reach them when I had some hip issues so I was thinking about for the future - I was told that if I move a plug, the whole thing has to be rewired back to the panel box. And the darn kitchen sink has to be approved by the UL LOL - who would think a kitchen sink could be dangerous.

I have Calacatta marble basket weave in my master bath (honed) and I have Carrara dog bone in my guest bath. This is both floor and shower floor. I absolutely love it and have been using it with no issues for over a year. My shower was built properly so there is proper slope so there is no water puddling anywhere. I also have a lifestyle that doesn't include washing dogs in my shower; dying my hair in my shower or cleaning off from my labors in a coal mine. I take reasonable precautions - e.g. I use shower gel instead of bar soap and I have products that are safe for ALL my surfaces. At the advice of Dragonfly, I bought MB Stone Cleaner which is also used for my mirrors and marble counter and a non-acidic toilet bowl cleaner from Majestic. Prior to my remodel I researched quite a bit and also asked friends with marble what it was like to live with marble. None of them had any problems or particular horror stories. They showed me a bit of etching (aka patina) but frankly it was hard to see unless the light was shining and I squinted. FWIW, honed is essentially etched which is why I opted for etched in the master which theoretically gets more use. I also went with honed for aesthetic reasons because it was softer looking. It's obviously a personal decision. When I was looking at various options in terms of counters and floors, I did look at the manmade stuff like Neolith, Dekton, quartz but they didn't appeal to me aesthetically as much as the "real thing". Other people could come to a different conclusion.