This is typical micrite/mudstone limestone. It doesn’t show any rock fragments, nor limestone grains. The stone was formed in quite deep, calm water, below wave action and probably even below storm wave action. The rock formation is therefore very dense, most likely the seawater was below 15 degrees Celsius during sedimentation, so aragonite crystals were formed. At lithification (stone forming) water was pushed out and a dense calcium carbonate was formed, with no iron content, very limited pore space, resulting in very solid rock with a very stable colour, which won’t be affected by ultra-violet, so no discolouration at later stages.

Stone was only studied using a hand lens, for detailed description microscopic study is required, no such study is needed as macroscopic study is sufficient to determine the nature of this material.