The opalized coloring on the ammonites is very reactive to light.
The predominant color is red with green being the second brightest/visible color in my opinion.
It is very addicting to lean them forward and back against the light.
They almost look like a spool of metal wires lighting up and fading away - just to come back on each time you change direction.
I currently have 3 sizes of opalized ammonites:
Small: 5 to 10 grams
Medium 15 to 30 grams
Large: 50 to 65 grams