This is a Mastodon tusk fragment I found this in a fresh water environment in early November.  It seemed relatively stable after cleaning, but over the past four months the uniform dark brown has taken on this mottled pattern.  I haven’t detected any instability, no flaking or crumbling, but it no longer sounds solid when tapped with a finger,  I assume cracks and fractures are propagating with progressive drying and differential shrinking.  I know modern elephant ivory can develop cracks