diamond block spawning on level 2 or 3

made a new world, with mostly defaults except with starter chest, notices some chunks not loaded but when I walked they didn't change. went into LAN with cheats, went into creative looked down and there was diamond blocks at level 2 or 3

Seed: 3018346697426790211
Co ords where I found it: -246,2,278

if you need more informationpleas email me at bradboy7@gmail.com