Most dense wet woods have a density less than ##1250 kg/m^3##
Less dense natural stones ##2400 kg/m^3##
Therefore a saline solution with intermediate density can buoyancy separate acorns from pebbles.
Wet acorns are not comparable to the densest woods.
The variation in density of saline solutions is linear with the increases in salinity, the saturation of salt in the water occurs after 35% by weight or approximately 350 g per liter, since the variation in the volume of the water to add salt does not affect too much to calculate, so densities of ##1300kg/m^3## are easily achieved, the temperature does not alter the results if you use sodium chloride.
For economic reasons it is better to experiment with dilutions of lower concentration ##200g/l## seems a good criterion
You could develop the following experiment
• In an empty container, place another container with a lid that closes it at the root, without modifying its capacity, fill the internal container with tap water, until one more drop overflows it and spills,
• Weigh about 100 acorns or any other quantity, put the acorns in the container, the water will spill, put the lid on allowing all the acorns to sink into the water
• Remove the container and weigh the spilled water,(discounts the weight of the container), the weight ratio of acorns to weight of water will give you an idea of the average density.
The saline solution should have a higher salt to water ratio than the previous ratio, so that you can achieve efficient separation.
$$ (\dfrac {weight\,of\,acorns} {weight\,of\,spilled\,water} -1) \cdot 1000 <x $$
## x ## = grams of salt per liter to dissolve
in case of negative, you could separate them with fresh water, or tap.