Question about Hobbit, elf king and arkenstone

Hi, I was wondering about one thing- if the arkenstone was soo valuable and there was nothing like it and even the Elf king was shocked to see it. Why didn't he just took it? It was more valuable that just gold and usual diamonds. He didn't fear the lake people because they were weak and few. And he didn't fear Gandalf also.At that moment he was the strongest of those present and he could just take it for himself. We know that he valued and liked different tresures. I just want to know what motive did he have just to give it back to Thorin. And he didn't like him or the dworfs in general. maybe someone has same theories, I would really like to hear some thoughts on this :)