do you mean the door threshold?
I used the same granite I used on my vanity top. in my hall I have old carpet that I need to replace. I agree that marble might be too much of a contrast in your situation and I would probably use oak.

I guess I don't have a picture showing my threshold either but I also used the same granite as my vanities, tub surround and shower door threshold. This photo taken from the doorway will give you an idea of the look.

I agree that in your case, matching the marble from your counter might look too stark against your slate floor so I'd probably go with a wood threshold that would match the oak floors outside the bathroom.

I used black Corian. I have Brazilian cherry hardwood in the bedroom and Seneca handmold in deep sea blue in the bathroom. I didn't want something that would detract from either material, and I think it sets off both beautifully.

I went with marble where it coordinated with the floor tile...wood where it didn't.

In our bath, we went with a strip of the accent tile from the shower--this was something our builder was doing in his spec houses and we really liked it. This was great because I never really liked the marble thresholds to begin with--they never look right to me, they stick up too high posing a trip hazard and they are so cold. Raised transitions also tend to collect dirt along the edge that can be difficult to remove/sweep out.

We used the accent tile transition technique at the doorways between the bedroom and the bath and the closet and the bath. Note that our transition was from carpet to tile in both. In our laundry, we did a very thin metal transition strip. Both transition options allowed us to keep the floor height at the transitions as level as possible reducing trip hazards and making the house more handicap accessible should the need ever arise.

Here's a pic:

I have black slate and chose a black marble pre made threshold I got at The Tile Shop. Here it is shortly after grouting the floor but before it was caulked at the junction with the tile.