I think Jean's example is most likely hippo ivory. See
here for some pics of hippo ivory that show similar patterns.
A quick search for "keris" and "hippo ivory" produced an item currently for sale in The Netherlands. I won't post the link for obvious reasons. But the description included an interesting claim about Bugis trade with Africa:
The Buginese type keris is one of the oldest known in Indonesia, The hippo ivory hilts were imported by Buginese sailors in Africa before the 15th century as described in Robert Dick Read’s Penjelajah Bahari.