Infrared Differences near Opportunity

Color enhanced infrared picture. Mapping of shown color to IR wavelength: red = 800nm, blue = 900nm Original data: Opportunity, Sol 616 Picture and discovery: Norbert Gasch

Next to socalled blueberries, those small bluish spherules with 2-6mm diameter, we see on pictures from Opportunity Rover some larger, irregular shaped gravel with similar bluish color.

Seen through infrared filters, those gravel stones, all equal in visible light, show obvious differences in color. Some show strong reflections of infrared light with wavelengths around 800nm, some other with 900nm, as shown in the image on the left.

The cause for these color differences is not known yet. On Earth, we can observe similar effects on infrared pictures of plants where this observation is used to characterize different types of vegetation by remote sensing though satellite infrared pictures.
The next picture shows the same area in visible light as a human on Mars would see it himself.

Update:

The same effect is visible on a picture from Sol 314:
Discussion about Sol 314 image.

Same color differences on "blueberries":
Discussion about Sol 645 image

Source of original data: NASA/JPL MER Raw Image Archive