Heat 1–2” water to a gentle simmer (not boiling) in the bottom section of a double boiler or in a 2 quart sauce-pan.
Place the top section of the double boiler over the bottom section, or fit a stainless steel or glass bowl (larger than the pan diameter) over the saucepan. Place the chocolate pieces into the top double boiler pan or the bowl.
Stir every few minutes with a nonstick silicon spatula until completely melted. Be sure not to allow any water to mix with the chocolate or it will “seize” and become grainy.
While chocolate is melting, prepare rack and sheet pan. Line a sheet pan with aluminum foil (or use a silicone baking liner in the pan). Place flat cooling rack into pan.
Remove pan/bowl of melted chocolate from heat and let sit a few minutes to cool and thicken slightly. Stir in pitted prunes until they are entirely coated with chocolate.
Using a dinner fork, scoop out prunes one at a time, letting excess chocolate drip back into the pan of melted chocolate, then place chocolate covered prunes on the cooling rack. Continue until all prunes are on the cooling rack and not touching each other.
Place rack in a cool location (or refrigerator) until chocolate is solid again. Remove “rocks” from rack with the tip of a blunt butter knife to avoid creating fingerprints in the chocolate.
Store in an airtight container in a cool place or in the refrigerator for around 30 minutes.