Sifting
With  Rubble in your Inventory and a Mineral Pan equipped allows you to use  Sift Rubble action on  The Shallows to wash your Rubble and reveal if there was  Any Raw Gem,  Black Sand, or  Goldflakes hidden within. Raw Gems have both a variable number of Hardness which determines how difficult that Gem is to cut on  Gemcutter's Wheel and a have a variable number of Carats determined purely on chance, which gives a bonus to the Cut Gem's Artifact stats and if over 1.00 carats, change that Gemstone's size. Finding Any Raw Gem during a  Blood Moon will instead give you a  Blood Shard. Finding Any Raw Gem while Sifting reduces a bit of Insanity.
Gem Hardness
Any Raw Gems you find while Sifting will have a Hardness which relates to its odds of successfully being cut on a Gemcutter's Wheel - the Harder the Raw Gem, the less chance of success. Hardness is determined at the time of Sifting based on a your  Mines & Mountains Proficiency and chance. Additionally, each different Gem Cut will add a fixed amount of Hardness at the time of cutting.
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The Precious which will crack Any Raw Gems you find into 
Fractured Gemstones.
