Over time more calcium carbonate accumulated around these nuclei in concentric layers, forming small calcareous spheres (less than 1mm diameter). Countless billions of these spherical sediments, called 'ooids' or 'ooliths',(what are 'ooliths'?) ultimately became buried and partially cemented together by more calcium carbonate, resulting in the oolitic limestone we now call Portland Stone. The process of lime scale build up in a kettle in hard water areas is similar.
Fortunately, the degree of cementation in Portland Stone is sufficient to allow it to resist the detrimental effects of the weather, but it is not so well cemented that it can't be readily worked (cut and carved) by masons. This is one of the reasons why Portland Stone is so favoured as a monumental and architectural stone.
The earliest known building to be constructed using Portland stone is Rufus Castle at Church Ope Cove, Portland. The original structure was probably built in around 1080, rebuilt in around 1259 and rebuilt yet again in about 1450 which is the likely date of the walls seen today.
Portland stone was used to build the Palace of Westminster in 1347, the Tower of London in 1349 and the first stone London Bridge in 1350. Exeter Cathedral and Christchurch Priory, also constructed during the 14th Century are built of Portland stone. The East side of Buckingham Palace, the official London residence of Queen Elizabeth II, including the balcony, was faced with Portland stone, first in 1854 and again in 1913. The Victoria Memorial is also made of it.