The stone was a consistent architectural feature in the house. The fireplaces and the exterior of this same curved wall were all clad in the same material. Sourced and install by Stone Mason Doug Burpee. We elected to make a feature of this bath by installing it on the interior too. Where the showers are, however, we went with metal tiles for easier cleaning.

How would you sink the tub like that in a small renovation?

This is a great look for the tub and wet room...The construction was made possible through a couple of of planning elements made easier as it was a completely new build:
1. The guest room was on the first floor, with an unfinished basement below.
2. We had the luxury of being able to sink the tub and build the appropriate structure to support it below, planning ahead to keep any mechanical issues out of the way.

In a renovation you would need to have all of these elements in alignment and be prepared for significant surgery to alter any existing structural & mechanical systems to build to required support structure and drain system..

Did they use a sealer on the stone in the bath area?

Typically we specify a non-enhancing top quality sealer for all stone, mortar and grout. Your stone supplier should recommend a high quality material.