The sightstones both give you a set of placeable "one-time use" green wards. You're only able to place two before the earliest one placed is replaced, however, but the amount you have is refilled when you return to the fountain. If you're a support, work towards trying to get it but don't hog your gold if you're back to fountain without enough to purchase the stones. If you're anybody else, purchase the one-time use ones and place them. The support isn't the only one who should be warding the map. --- http://i.imgur.com/yIM1r.jpg
Sightstone is a waste of gold. The wards don't last as long, you can only place 2 at a time, and they're only green wards.
I stopped getting it because I found myself buying regular wards on top of the sightstone when I played support. Maybe if ruby sightstone gave pink wards it'd be worth it. --- Go to work, send your kids to school, follow fashion, act normal, walk on the pavement, watch TV, save for old age, obey the law, repeat after me: I am free.
Its something like 700 gold for sightstone. At 75 gold per ward, thats 10 wards you have to place to make up the cash. Typically only the support places that much in a game.
It also gives a ruby crystal's worth of health as well. Current cost is 950, then you subtract the ruby crystal's amount and you're left with 475. 475/75 = 6.3~, meaning you need to place 7 wards for it to be worth it. If you're a jungler or a support, then this item is still a great one. --- LoL IGN: Mr Microwave