My thinking is that if half are lost but three times more is processed than it is still a success?
I tried it once but only got tons of spinel and a fair bit of cassiterite but I think water pressure was too low.
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I’m no expert on gems, but my feeling is that all the heavy “black sand” minerals will have a significantly greater density than the gemstones.
So the riffles will get clogged up and any gems likely to skip over the top.
But easy experiment to try!
Perhaps give it a go for 20-30 min and post the results 
Cheers
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I tried it once with a little 6" backpack sluice, and like Drystone said, the traps were completely full of fine black spinel after about 3 shovels of gravel. I would imagine that you’d need a really big sluice with really deep traps to make it work.
Does anyone have any idea how the old timers separated the tin from the spinel in their long tom sluices?
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Tin is about twice as heavy as Ali el, so even though your riffles may look full, they’ll keep getting filled with tin which will be displacing the spinel.
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Looks possibly like a nice piece of cassiterite, if so quite river worn
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I sluice quit often for smaller gems ie saphire etc.
I keep the sluice on a minimal angle what I have also found successful is I place a pan at the bottom. That way anything heavier that washes down gets caught in the pan sinking to the bottom whilst the lighter stuff washes out
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