Supernova Gemstone

{0}: If a spell you control or an ability of target permanent you control would cause or trigger an
opponent to spend mana, prevent that opponent
from spending mana in this way.
If you have 5 Life or less, Supernova Gemstone
cannot be countered and has hexproof.
No one knew if the stories were true, that it was forged
from the last moment of a dying star or if it contained
that last moment within itself...