Silk and linen are the traditional materials for the 12th anniversary.
For a romantic yet still traditional twist on silk, try something along the lines of these silk roses.
Silk also lends itself well to a wide variety of luxurious clothing gifts such as robes, shawls, scarves, shirts, skirts, dresses, and lingerie.
Practical gifts like high-thread count bed linen, tablecloths, towels, cushions, and other home decor would be completely appropriate as well. This could be a good opportunity to do some home redecorating to freshen up the mood of your bedroom.
If you know for a fact that your wife enjoys new lingerie, this is a great opportunity to buy her something gorgeous. But our word of advice is that, if you are unsure of her reaction to lingerie, then get her lingerie PLUS a more traditional gift, since some women do not care for lingerie. But you never know until you try! :-)
If you would like to give jewelry, you could opt for pearl which is the alternate modern material. Also, great choices are jade and opal which are the alternate modern jewelry materials for the 12th anniversary.