Breed with a Ditto.
It will be luck because i have 33 Eevees and 27 of them are boys. Boy Eevees are more common.
No, unfortunately, you can only get more eevees in crystal version is to breed one.
Girl Eevees are rare. Because more than 95% of the time boy Eevees hatch instead of Girls. Out of my overall 60 Eevees, 55 are boys. But it is possible.
Eevees
You give it a fire, water, or thunderstone. If you want an Espeon or Umbreon, you will have to trade it into a ruby, emerald, or sapphire game. If you want to get all the Eevees, then you should breed your pokemon on island 4.
trade
Yes. All eevees can matewith all other eevees in diamond, pearl, and platinum.
Yes they are and in Evolution too.
umbreon
Get a ditto and a leafeon and you will breed eevees. Then evolve them with a leaf stone. You can breed eevees with any evolved forms. It is not possible to hatch eggs as leafeons.
No.
Yes, if it is pure natural sapphire it will be worth more than a diamond.