Based on the description, the rock is likely a fine-grained limestone. Its light grey color, hardness of 6 on the Mohs scale, and effervescence with dilute hydrochloric acid, which is a reaction with the calcium carbonate (CaCO3) content, all point to limestone. Limestone typically forms in shallow marine environments from the accumulation of the skeletal fragments of marine organisms like corals, foraminifera, and algae.
The sample is light grey, fine-grained, with a hardness of 6 on the Mohs scale, and effervesces with dilute hydrochloric acid.
You are a professional geologist. Analyze the provided rock sample and determine its type and possible origin.