Heart Scales are really useful. You can dig them out in the underground. Go to Pastoria City and go south from the Pokemon Center. Next, when you see the water go to the house near it. The guy in there will teach your Pokemon moves that it forgot. This is useful if you need a combination but the move isn't a TM and your Pokemon forgot it. For Example, if you want your Gengar to know the combo Hypnosis, Nighmare and Dream-eater and your Gengar already forgot hypnosis then you use a heartscale.
You can go underground to dig up the heart scales or just caught a Luvdisc at Route 224 and at the Pokemon League with the super rod and you have like 5% to find him/her and one more thing you might have a chance getting a heart scale of 50%.
you can find them underground
Go to Pastoria city if you have Diamond, Pearl, or Platinum and give to this guy and he'll make your Pokemon remember moves. If you have Soulsilver or Heartgold, then go to Blackthorn city and go to the move deleter's house. The guy that will make your Pokemon remember moves is wearing a pink shirt and blue trousers.
you use it on move tutors to teach your Pokemon moves
Canalave City and Pastoria City
Shaymin sky form is only in Pokemon heart gold,soul silver, and Pokemon platinum by showing the flower shop lady a shaymin.
There is no king in Pokemon pearl
You can find them in the Underground
You can get Heart Scales in Pearl from digging in the Underground and (P.S. This is for after you get the National Dex) from wild Luvdisc.
Some people may give them to you or you dig underground.
Heart scales are obtainable at the pokeathlon prize exchange booth. The pokeathlon is north of goldenrod city, in the Johto region. Hope I helped!
get an explorer kit from the old man in eterna city and dig up heart scales.
To find heart scales on Pokemon diamond/pearl you need the explorer kit to go underground. once you are underground look for cracks inthe wall, and eventully you will find a heart scale in the cracks.
To learn moves you forgot or didn't learn(exept TM & hm)
try trading heart scales to teach your Pokemon new moves to the guy in pastoria city.
The move tutor is in the South-east of Pastoria City near water. You will need heart scales for the guy to be able to teach it to you. You can find heart scales in the underground.
If you dig underground and dig in the walls you can get heart scales.
Heart Scales can sometimes be found on Luvdisc when they are caught.
on Pokemon diamond and pearl you will need the explore kit to go underground then after mining the walls for loot there is a chance you may find a heart scale also fosils and evoloution stones