Looks like oxalate/calcium resides from the plant. You'll see them (mostly underneath leaves) occasionally, especially when healthy and well watered.

It's not harmful or unhealthy.

Thanks for your reply. I looked up Calcium Oxalate and the wiki article says its poisonous and possibly fatal? Are we taking about the same calcium oxalates here. Because that sounds it bit unsettling.

"Calcium oxalate is a poisonous substance that can produce sores and numbing on ingestion and could even be fatal."

http://en.wikipedia.org/wiki/Calcium_oxalate

http://www.helpinggardenersgrow.com/2013/01/spinach-gritty-truth.html

Grabbed from another posting on GW :)

Technically yes, but you're not going to get that huge of an amount from a normal serving of veggies. It's more of a concern to those who are sensitive/allergic to it...and even then it's more of a skin irritation or upset stomach thing for most people who are sensitive.

Beyond that, even if just for taste and texture issues, you'd be washing those crystals off your plants before eating them. It's a bit like eating sand.

There's a kinda scale (not a literal 1-10 or whatever scale) of how severe different types of plants produce this...and spinach is really low on that scale. Even when it doesn't produce physical signs of crystals outside the leaf stoma it's still a part of the makeup of the plant.

Excellent. thanks!