Same place in regular R/S/E. (I don't know off the top of my head)
serebii.net is a reliable place to find info on pokemon games. AWESOME pokedex.
Star of India: A large, caochon-cut grey blue star sapphire,slightly oval shape
Yes, you can get the Gemstone Ring: Platinum Oval Sapphire & Half-Moon Diamond Ring with a ruby instead of a sapphire.
Hapiny + Oval Stone = Chancey
The Oval stone evolves Happiny into Chansey.
Chansey evolves from an Oval Stone.
happini can evolve with a oval stone
Do you mean what is an oval stone? Well, an oval stone is an item that when held by a Happiny during the Daytime and leveling it up evolves it into Chansey.
Happiny---->Chansey using the oval stone.
An Oval Stone is a held item that makes certain Pokemon evolve
The only Pokemon that can be evolved with an Oval stone is a Happiny into a Chancey.
Happiny evolves into Chansey with an Oval Stone.
You can evolve happiny with an oval stone after its friendship is high