Heartstone
Artifact
Activated abilities of creatures cost 
less to activate. This effect can't reduce the mana in that cost to less than one mana.
"Finding a true heartstone is even harder than finding a true heart." —Hanna
John Matson
Stronghold (STH)
• #134Rulings
Activated abilities contain a colon. They’re generally written “[Cost]: [Effect].” Some keywords are activated abilities and will have colons in their reminder text.
(wotc • 2016-06-08)
Can’t reduce Snow mana costs.
(wotc • 2006-07-15)
It will never reduce any colored mana portion of an activation cost.
(wotc • 2004-10-04)
It will not add a {1} to abilities with no generic mana in their activation cost.
(wotc • 2004-10-04)
The cost reduction can be applied to additional costs.
(wotc • 2004-10-04)
Rulings
Activated abilities contain a colon. They’re generally written “[Cost]: [Effect].” Some keywords are activated abilities and will have colons in their reminder text.
(wotc • 2016-06-08)
Can’t reduce Snow mana costs.
(wotc • 2006-07-15)
It will never reduce any colored mana portion of an activation cost.
(wotc • 2004-10-04)
It will not add a {1} to abilities with no generic mana in their activation cost.
(wotc • 2004-10-04)
The cost reduction can be applied to additional costs.
(wotc • 2004-10-04)