When in doubt, whether you’re standing in front of a display at a home improvement store or a furniture shop, the best option, materials wise, is always the natural one. Wood, stone, wool, and cotton, for example, have a better effect on how your brain works and how calm/unstressed you feel that plastics and polyesters, for example.
When we’re near natural materials our brains process incoming information more effectively and we feel calmer—this is one of the central tenets of biophilic design and has been confirmed by study after scientific study, in labs and in the actual real world.
Much of the research done with natural materials has been done with wood, and particularly with wood whose grain is visible. Research shows that seeing the grain specifically has all sorts of benefits mood and mental performance-wise, so, sorry, ebony with its dark, dark finish isn’t the way to go. Scientists have learned that the best wood finishes of all, from a brain perspective, are those that are warmer, for example, oak. Seeing wood grain can lower blood pressure and heart rates and stress while elevating mental performance in general and also creativity.
Too much wood can definitely be, well too much, however. Melding the results of multiple studies together indicates that its best if no more than 45% of the surfaces in a room (so, that’s the floors, the table tops, etc., all added together) have visible wood grain.
It’s harder to imagine potentially overdoing the amount of stone or natural textiles in a room. Slate, marble, etc., are heavy, and apt to be used only on sections of floors, and countertops, and chimney mantle fronts, for example—unless they’re a beautiful amber or agate or other material that can be cut into beautiful thin sheets and backlit—and those ambers, etc., are so phenomenally expensive that you’re unlikely to be available to afford it, except if you’re well established royalty (remember the Russian czar’s amber room?).
Textiles are deployed in use appropriate ways, whether that’s “hardened” for carpeting applications or in softer, less processed states as pillow tops or upholstery or echo busting wall hangings.