You have to combine three small Earth (brown) gems with one of the purple ones. However, there is a bug that keeps it working on all the sites on which Arcuz 2 is hosted besides for Gamesting.
There are a very few people who have allegedly been able to complete the quest on other sites like Armor Games but almost every single person who has played it on any site but Gamesting can not do it.
Basically, if you want to complete the Elf Poem quest, you will either have to start a new game or figure out how to moved your saved data to that site. I'm not sure how you do that but there is supposedly a way. The good news is that the Elf Poem quest is not necessary in the game but if you want to get that achievement you will have to restart or move the saved data.
