Didymium filter
From The Glossary of Digital Photography
Also called an intensifying filter because it intensifies reds, oranges, and browns without affecting other colors. This filter can be useful in landscape photography, for example. When the subject falls within the above-mentioned color groups, the image will be more saturated and have more contrast. A didymium filter contains two rare earth metals: neodymium (Nd) and praseodymium (Pr).
