Minerals often grew within ammonite shells and are one way in which they were preserved as fossils. Minerals can be in solution in water (in the same way as sugar can dissolve in tea). When water containing minerals entered the ammonite shell the slightly different chemical conditions in the shell caused the minerals to leave the water - to come out of solution - and line the surfaces of the shell, lining the shell from the outside surfaces and growing in towards the middle of the shell.
Ammonite shells are divided into chambers, with only a tube called the siphuncle linking those different parts of the shell. On some ammonites you can see how the individual chambers are separately lined with minerals! Sometimes the mineral growth never reached the middle and we can see the crystals lining the shell with a hollow in the middle. You can see that on the picture of this ammonite: http://www.nhm.ac.uk/nature-online/earth/fossils/ Ammonites can also be filled in with sediment - mud - instead of crystallised. Often, the outer chamber of ammonites filled with mud and the other became filled with crystals.
The dull white surface on your ammonites is where the original aragonite surface of the ammonite shell is preserved!