The kaseki is used to create the seasinger kiba and the seasinger makigai. It can also be used to recharge these weapons once they've fallen below 50% charge. The (standard) voyage rolled depends on how many tsubas/kasekis and tetsu/seasinger weapons you own in total. If you have more of one, then the voyage will be for the one you own the least of, otherwise it's 50/50. A Gemstone kaseki can be used to repair a seasinger kiba by 50%, or a seasinger makigai for 95%.