I have marble all over my 3-story historical 120 year-old townhouse. I have large Carrara tiles, with dark diamond accents in the bathrooms, and also have pale travertine as my kitchen counters.
It all came/was polished... and honestly looked contractor grade... even though I paid more than I should have, I was disappointed. Over the years the shine faded, especially in the traffic areas. My darling husband had also created a few stains around the toilet (!).
I had to do something... so:
I applied oxy/bleach and the stains disappeared. I also used a oscillating sander with 220 grit paper to sand down the shiny spots. I then sealed the entire area with sealer (just once). Worked like a charm.
Someone also set a lemon slice on my counter top which sat overnight. I now have a perfect lemon-shaped impression etched into the travertine. I was initially upset, but it actually looks very pretty, like a fossil.
I would NEVER use a man-made product. They don't look the same with a vintage/antique product. Every time you walk past that bathroom you'll regret what you did.
If this were mine to decide, I'd go to an architectural salvage yard and buy a piece of vintage marble. If this is a sink, you'll have to have it cleated (attached with a bracing cross board between studs) to the wall to remain stationary. Remember this when you measure to cut any marble/stone. The cabinet should be installed before you do any measuring, as you need to allow for the back wall gap or even a back splash (a good idea with a sink). If you are DIY-ing this, the back splash needs to be attached to both the cabinet and wall with no movement.
And don't limit yourself to just Carrara. There are so many gorgeous varieties of light-toned marble with coloured veins that can accent that lovely cabinet. With such a small area, you can go a little more extravagant and dealer always have a scrap/damage section where you can really talk down price.
Q