package com.sapphire.world;

import java.util.Random;

import com.sapphire.items.blocks.MainBlocks;

import cpw.mods.fml.common.IWorldGenerator;
import net.minecraft.world.World;
import net.minecraft.world.chunk.IChunkProvider;
import net.minecraft.world.gen.feature.WorldGenMinable;

public class WorldGenSapphire implements IWorldGenerator{

public void generate(Random rand, int chunkX, int chunkZ, World world, IChunkProvider chunkGenerator, IChunkProvider chunkProvider){
switch(world.provider.dimensionId){
case -1:
generateNether(world, rand, chunkX * 16, chunkZ * 16);
case 0:
generateSurface(world, rand, chunkX * 16, chunkZ * 16);
case 1:
generateEnd(world, rand, chunkX * 16, chunkZ * 16);

}

}

private void generateSurface(World world, Random rand, int chunkX, int chunkZ){
for (int i = 0; i < 100; i++){
int randPosX = chunkX + rand.nextInt(16);
int randPosY = rand.nextInt(64);
int randPosZ = chunkZ + rand.nextInt(16);

(new WorldGenMinable(MainBlocks.sapphireOre, 10)).generate(world, rand, randPosX, randPosY, randPosZ);

}
}


private void generateNether(World world, Random rand, int chunkX, int chunkZ) {

}

private void generateEnd(World world, Random rand, int chunkX, int chunkZ){
for (int i = 0; i < 100; i++){
int randPosX = chunkX + rand.nextInt(16);
int randPosY = rand.nextInt(64);
int randPosZ = chunkZ + rand.nextInt(16);

(new WorldGenMinable(MainBlocks.sapphireOre, 10)).generate(world, rand, randPosX, randPosY, randPosZ);

}
}

}

This maybe incorrect because last time I did ore gen not in the overworld was 1.4 but from what I remember worldgenmineable finds stone and replaces it with the ore that is specified. In the end however there is no stone to replace so nothing happens. So you will need to create your own worldgen class to replace endstone with your ore.

You can replace end stone with WorldGenMinable, look at the constructors.