My Lands: Black Gem Hunting

On the server there are the ruins?

yeah

I have found a handful of ruins so far... what is point of this post?

Do ruins go away, or are they destroyed? I was attacked by monsters coming from a ruin, so I explored the area, and there was no ruin

Ruins go away naturally after a time (it varies), or they can be destroyed by players; either of those might have happened here.

Most of what you're likely to need to know can be found in the relevant Encyclopaedia article.