This was done in beta version of RuneScape and method may get changed before released in game. Gem rocks don't have rockertunities and don't give geodes. So they are not the highest mining xp/h but good combination with crafting xp/h. I got 100k crafting xp/h and 120k mining xp/h with 99 mining, using Rune pickaxe +3, mining urns and magic golem outfit.