i think there are two kind of diamond shape object in the timeline panel.
white diamond and inside-transparent diamond.
sometimes it switches to white (or inside-transparent), and i cant figure out what are they represent for..
is anybody have a cue??
A solid 'keyframe marker' simply records the current 'property panel options' state, at the position of the playhead on the Timeline.
If Auto-Transition is OFF the next recorded state along the timeline, the keyframe will be hallow - which simply means no transition occurred between the previous recorded keyframe. In appearance, when you playback the animation, the the movement will be 'instant' or abrupt from one keyframe to the next.
See illustration below.
1. Auto Keyframe/Transition set to OFF, there is no keyframe recording.
2. The moment Auto Keyframe is enabled, but with Auto-Transition OFF, you get a solid keyframe.
3. With #2 options still in place, the next playhead position will track the movement, but will be respresented as a hallow keyframe.