Gems?

Hey guys I had a question about the gyms I just got three gems. looks like a fire wind and water. I'm trying to figure out how to use them can anybody tell me how they are used?

Mandem gemas. .....

It's for upgrade...each item has a cap lvl with between 50-120 based on rarity. With Stones u can break the limits. Magic stones for ring and necklace iron stones for helm shoulder and no element weapons and the others for element weapons

What does the "cull the weak" stat do?