Nataly, Without having that stone in my hand, I can't tell you what you need to know. This is why it's so important to have an experienced qualified tile contractor on site to advise you regarding material selections for a specific application. I put my hands on the products and years of experience tell me a lot. I can "feel" the difference between a quality marble and a more porous, less quality marble. It's about density. I lift it up to the light and look for fissures that weren't otherwise as apparent (another indication of density - with a softer body it is harder to see until lit from behind). I also look at clarity. All marble contains iron deposits that can yellow/oxidize over time, some more than others. Moisture can exasperate this process. What's the substrate? Shouldn't generally be an issue for a fireplace if the proper backer board and approved methods are used. If I approved the marble selection I'd probably use Laticrete 4XLT over the substrate (not a foam backer board) and I would use an appropriate grout joint and wait a few days for the the setting materials to cure before I grouted so as not to trap moisture (I'd probably use Laticrete Select grout, but other unsanded grouts are approved and a pro has his or her preference). Now, as for quality differences in products from various suppliers. Yes, that is a real thing. Mining varies and processing varies and "made in China" can have an impact in my experience. But that's why you hire a qualified tile contractor who can guide you. And yes, use only appropriate cleaning materials for marble once installed.

Most likely the cracks are due to the house settling over the years. If house is on clay soil it may have dried out too much. Watering around the founation on regular basis will help eleminate the shifting of a house.

Thanks for everyone’s input. I do believe that floral is quite correct about what it is. I’ve read up on it and was a little concerned about it being toxic to horses as I did have (until 3 weeks ago) two horses that could access it. Both horses were/are older horses and the 26 year old Morgan died suddenly 3 weeks ago but he had been diagnosed with equine Cushings so I’m pretty sure that is what he died from. His death showed no signs of poisoning nor did he exhibit any signs when he was alive. Plus, there was no sign of ”the plant” having been grazed. My other horse, the American paint, Chex, was moved to a neighbours shortly after Shadow died and I don’t plan on keeping horses any longer so I’m not concerned about it being toxic to horses. I’m waiting for it to flower and then I will post the photo. beckysharp, I got my nine holes of golf in this morning and plan on hunkering down in the shade tomorrow.🌞

Thanks palimpsest. Instead of redoing the whole thing, what do you think of cutting out a window or several access holes on the side of the vanity (that side has no drawers) along the bottom to create access to caulk properly along the shower base? Yes there would be unsightly hole(s) on that side of the vanity now (only visible if taking a shower), but at least the base would be caulked on the outside? I could slide a white 1x4 between the glass and vanity after to hide the gap/holes, and just remove it for routine cleaning/maintenance. Unless anyone has ever seen a super long caulking gun tip to reach back there.