There is one other note for the coloring of ivory. As Spiral says it depends on environment and one environmental factor is the amount of exposure to sulfur.
Sulfur is mostly what colors ivories. For example elephant ivories in the presence of sulfur will turn yellow to orange to brown. IT is always in the air, but heat will accelerate the chemical reaction as the ivories "breath" in the sulfur already in the air.
By the way, Jens I always love your pictures - and am
insanely jealous.