What is the likelihood of finding a fossil in the Underground?

After the National Dex.

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Each item has a respective appearance rate, which depends on game version (DP) or the player’s Trainer ID number (Pt) and on whether the player has obtained the National Pokédex. Appearance rates work like a lottery system, where the rate is the number of lottery tickets an item has in the pot. For example, an item with a appearance rate of 15 is 3× more likely to be selected than another item with an appearance rate of 5. The exact likelihood for an item to be selected is its appearance rate divided by the sum of all appearance rates in the same list.

In practice, item rarity is more complicated. First, an item is selected from the appropriate list, weighted by appearance rate. Next, an attempt is made to place the item in a random spot on the mining grid. This attempt fails if the item would partially hang outside the grid area or overlap with a previously-placed item. If this happens, the process starts over by selecting a random item from the list again. Large items like the Root Fossil will fail to be placed more often than small items like the Heart Scale, and therefore they will be less common than their appearance rate alone would indicate.

The rates below assume the player has received the National Pokedex. These numbers arenotpercentages.

Helix Fossil: 12Dome Fossil: 1Claw Fossil: 12Root Fossil: 4Old Amber: 5Armor Fossil: 0Skull Fossil: 12

Helix Fossil: 4Dome Fossil: 13Claw Fossil: 4Root Fossil: 12Old Amber: 5Armor Fossil: 12Skull Fossil: 0

Of these, only the Armor and Skull Fossils can be found before obtaining the National Pokedex. When this is the case, the Skull Fossil’s value is 25 in Diamond and Platinum’s odd IDs; the Armor Fossil’s is 25 in Pearl and Platinum’s even IDs.

Underground (Bulbapedia)

A lot higher than before you get it, but I find one every 7-10 times I dig.

Armor Fossil - Revives into Shieldon -Uncommon(Pearl Only)

Claw Fossil - Revives into Anorith -Rare(After National Dex)

Dome Fossil - Revives into Kabuto-Rare(After National Dex)

Helix Fossil - Revives into Omanyte -Rare(After National Dex)

Old Amber 1 Revives into Aerodactyl -Rare(After National Dex)

Root Fossil - Revives into Lileep -Rare(After National Dex)

Skull Fossil - Revives into Cranidos -Uncommon(Diamond Only)

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