Diwal is a sand-burrowing clam collected by divers from sandy-muddy bottoms of inter-tidal areas in coastal waters, 6 to 8 meters deep. They are also gathered by trawlers which are destructive to their habitat. When I was a kid, diwal was abundant and inexpensive---it was a favorite pulutan (finger food). But a few years ago, diwal was considered almost endangered due to indiscriminate fishing methods used by some harvesters, and harvesting was banned. The clams are now available but in small quantities, making it an expensive shellfish. My mother bought it at P250 a kilo and it could be a bit cheaper when in season. When I was in Negros last year, my cousin bought diwal for me at P400 a kilo---and it's sold at P500 a kilo in restaurants.
Not many folks know about great-tasting diwal unless they've been to parts of Western Visayas. In Valladolid, my mother's hometown, there are vendors selling diwal by the highway when the clams are in season, between May and July. And the buyers who stop by are mostly tourists and city-folks...apparently, the locals couldn't afford them anymore.
I like to keep its natural briny and sweet flavors so I prefer my diwal grilled. We also saute diwal in garlic, sweet onions and ginger, or steamed in lemongrass and ginger---incredibly delicious!