What the hell does one buy an 85-year old woman for Christmas? This was much easier when I was a little kid and could just make my grandma some sort of craft.
What are her hobbies? Does she do any gardening? There are some lovely gardening kits out there. I can point you in the right direction if you're interested. If she likes to cook, you could get some herb garden type stuff and a nice recipe holder, or blank cards and a cookbook and tell her you want her to preserve some of her classic recipes. Other cooking related things would be heart-shaped measuring spoons (I love my set), a tea set (teapot and some nice tea) if she's into that, etc.
You could make her a family album on snapfish or something, elderly people like memories and there are some really nice options that she might find nice. If she likes to reminisce, you could get her something like
this "story of a lifetime book" but that's only for a specific type of person. I have more super sentimental gift ideas but I won't post them here unless you need them.
You could do what I got for my person-who-is-like-a-grandmother and find out if there are any older movies or songs/artists that she likes and get them on DVD or CD for her. I found some stuff that she thought was impossible to find and she loved it!
Other "stuff" ideas are cozy things, like a nice throw, slippers, candles and such for the home (carefully chosen, not randomly picked!). A nice set of gloves and a scarf!