Can't pick up stone from minmus

[Diacasta]

I took a contract that ask me to collect some "green sandsone" (i think is the correct translation in english) from minmus, i sent a kerbal there, found the stone but i can't pick it up.....i have not the option in the menu ? Any clue ?

[AHHans]

Your Kerbal has to stand at certain positions relative to the sandstone. Walk around it and on top of it, and at some point you should get the option to pick it up.

[JoeSchmuckatelli]

It's on the 'left click menu' just like "take off helmet"