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The size of the seed crystal shouldnt matter too much. The key should be very slow formation (control the rate at which water evaporates to slow it) and ensuring the container you use has no nucleation sites other than the starter crystal.
Of course you also need to make sure your seed crystal is itself a perfect single crystal and not multiple crystals.
Rock salt, is usually very far from pure, particularly the kind used for melting ice. You need pretty pure salt to create nice crystals. That said if the rock salt were pure it would work as good as any. Once its dissolved in the water salt is salt if its pure. The seed crystal wont matter so much if its from rock salt or small granualted salt if its a single crystal.
When you have a collection of chemicals they may or may not form crystals. More often than not though non-pure chemicals form smaller crystals, if they form crystals at all.
@freemo @zleap @piggo
Either way, it reminds me of this
https://www.amazon.com/National-Geographic-Mega-Crystal-Growing/dp/B01C3DSKTY/ref=asc_df_B01C3DSKTY/?tag=hyprod-20&linkCode=df0&hvadid=242166129524&hvpos=&hvnetw=g&hvrand=6829070533420193425&hvpone=&hvptwo=&hvqmt=&hvdev=m&hvdvcmdl=&hvlocint=&hvlocphy=9032467&hvtargid=pla-505056962157&psc=1