List of Items You Can Dig Up Underground - Guide for Pokemon Platinum

This FAQ has a list of all of the items I know of so far.

SKULL FOSSIL:CRANIDOS

HELIX FOSSIL:OMYNITE

CLAW FOSSIL:ANORITH

ROOT FOSSIL:LILEEP

DOME FOSSIL:KABUTO

OLD AMBER:AERODACTYL

MIND PLATE

FIST PLATE

INSECT PLATE

DREAD PLATE

ICICLE PLATE

IRON PLATE

DRACO PLATE

ZAP PLATE

STONE PLATE

SPOOKY PLATE

SPLASH PLATE

EARTH PLATE

FLAME PLATE

BLUE SHARD

GREEN SHARD

YELLOW SHARD 

RED SHARD

FIRE STONE

WATER STONE

THUNDER STONE

LEAF STONE

SUN STONE 

MOON STONE

DUSK STONE

DAWN STONE

SHINY STONE

EVERSTONE

LIGHT CLAY

IRON BALL

HEART SCALE

DAMP ROCK

HARD STONE

RARE BONE

STAR PIECE

HEAT ROCK

ICY ROCK

SMOOTH ROCK

TOXIC PLATE

REVIVE

MAX REVIVE

BLUE SPHERE

RED SPHERE

GREEN SPHERE

PRISM SPHERE

PALE SPHERE

ODD KEY STONE

That is all for now hope you enjoyed the FAQ.