I should mention our bathroom is very similar to this one, except it is wider so we will have a 16" wide shelving area to the right of the shower. Also using a white tub.

Some slate looks like that but it depends where it is quarried from.

in my town it is blue, a hundred miles from here it is almost pure black due to the graphite content, but i have a house that has tan, blue with red streaks in its tiles and its slate still.

Maybe check into various colors of slate?

Slate is also very cheap for real stone and it’s textured surface will not be slippery in a wet bathroom.

You might be able to get it from the source of the bathroom - she doesn't say on the post where it is from, but looks like she gave it to people who messaged her

ooh interesting, thanks! I am not on instagram so I'll see if my wife can track her down.

I think they do not go with the shower walls anyway.

Thank you barncatz! Unforunately I now realized they are limestone; based on previous research I decided limestone was not what we wanted due to maintenance concerns and needing to stiffen the floor even more.

There are plenty of materials that can give that rustic look without the high maintenance.

A slate look. https://www.daltile.com/products/Stone-Look/Ayers-Rock/Majestic-Mound

https://www.daltile.com/products/Stone-Look/Slate-Attache/Meta-Light-Gray 

https://www.daltile.com/products/Stone-Look/Delegate/Off-White

A brick look. https://www.daltile.com/products/Stone-Look/Brickwork

Travertine look. https://www.daltile.com/products/Stone-Look/Archaia/Province-Grey 

Etc