According to snopes.com: “The use of finely-ground glass secreted in food is often mooted in murder mysteries and idle gossip as an effective technique for poisoning the unwary. Simply crush some glass into a fine powder, surreptitiously add the pulverized fragments to something your putative victim is about to ingest, then sit back and wait. It may take more than one glass-laden feeding, but eventually your target will fall to the floor, writhing in agony, before dying a grisly and painful death — and better yet, he’ll expire in a manner which will baffle all but the most experienced of medical examiners.
“Although the use of ground glass as a murder weapon is usually described as a method of “poisoning,” the ostensible effects of the substance when employed in the fashion described above are not poisonous in the usual sense of the word (i.e., it does not bring about illness or death through chemical means): the glass supposedly works its fatal consequences by chewing up the gastrointestinal tract of the hapless victim, causing him to bleed out (in some versions, eventually spouting blood from every opening in his body). In earlier times, however, powdered glass was believed to be poison in the ordinary sense of the word (i.e., it killed through toxicity rather than by cutting up one’s
innards).
“However you describe it, though, the bottom line is that the “ground glass in the food” killing technique just doesn’t work. The primary problem is that people really don’t care to eat food full of hard, gritty nuggets, and they really don’t like chewing on shards of glass that cut up their mouths. So, in order to make your glassified food palatable enough for your victim to eat it, you have to grind the glass very finely.
“If you really want to use glass as a murder weapon, your best bet is to pick up a large shard and stab your victim with it.”
So back to your dog. If they eat finely ground glass, they’re probably fine. But don’t let them eat large shards of glass. Bad dog.