It’s my summer vacation and I’m at the beach (Fire Island, in Long Island, NY to be specific.) Last night, while walking on the beach at night, my friends and I happened to notice that whenever we took a step, the sand beneath our feet sparkled and glowed like electric lights for a brief moment. It was like the sand had disco lights in them. What is the effect and what causes it to happen?
Probably not triboluminescence - more likely to be glowing dinoflagellates.
I doubt it. I grew up in Long Island and have never known there to be glowing dinoflagellates there at all. The Caribbean, sure, but not as far north as LI. On the other hand, I’ve never seen glowing sand in Long Island either 
I’ve seen glowing dinoflagellates in the water off Rhode Island. Not in the concentrations you see in tropical waters – just isolated sparks.
And I’ve seen them off of Woods Hole, at the SW corner of Cape Cod, so it seems possible on LI.
Bioluminescence occurs in a wide variety of marine microorganisms. It is certainly found commonly in temperate waters, both in the Pacific and Atlantic. Dinoflagellates are the frequent cause of glowing water in the surf or on the beach, and are certainly found in large numbers in times off of NY. (ever hear of “red tide”? Dinoflagellates.)
Off the coast of BC, I’ve seen bioluminescence on the beach that is very bright - and in the water, so noticeable that you could actually see fish swimming by leaving long glowing trails. These times always coincided with a red tide. The water in the daytime was so murky with microrganisms that when diving in it, visibility was only a few inches.
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This might help sort things out: was the glowing sand wet or dry? If wet, I’m betting on the dinoflagellates, which are sea creatures. I may be wrong, but it seems unlikely to me that triboluminescence works when wet.
I used to have a large fresh water aquarium and I occasionally treated the fish to a meal of krill. I got the krill at work for free in frozen blocks, I think it was Antarctic krill, about 3/8 to half inch long, small stuff.
Anyway, I thawed the krill by running warm water over a little frozen piece. And once I was walking though a darkened kitchen with my strainer full of krill and a little juice dripped on the carpet.
When I was done feeding the fish I walked back into the kitchen to put the strainer in the sink and saw all of these pretty blue splotches on the floor from the krill juice I had dripped.
I was amazed. It was glowing in the darkened room like it was radioactive. Not reflecting, but actually glowing.
Man, some people have all the good benefits. Grumble, grumble, stupid health insurance and sick days, grumble.
Being a Rhode Islander myself I can attest to seeing massive amounts of glowing dinoflagellates at times at night. The most ever being one night looking off the cliffs at Fort Wetherill. The ocean was aglow with tiny sparkles everywhere you looked. It was quite amazing.
EDIT: Oh yeah, and I would bet that’s what the Op is seeing. A quick Google and you find glowing dinoflagellates as causing sand to glow when you step in it.
I grew up in the northwest corner of Washington State, which is noticeably further north than LI. I had never seen the bioluminescence from dinoflagellates that lots of other people said they saw at the beaches (unless perhaps when I was too young to remember it) until finally a few years ago I was at the beach at night in the summer and saw it.
It’s a common enough thing when the conditions are right, but you have to be there to see it. I don’t know if you can find them in the waters of NY, but there’s no impediment to them being that far north as far as I can tell.
ETA: If it glowed for a bit rather than just sparkled I’m guessing it was dinoflagellates, because the glow sticks around for a bit after you step. If it was triboluminescence I’m guessing it would be a sparkle that would exist only while you were compressing the sand.