Andrew Trundlewagon's photos with the keyword: silica

Two forms of chalcedony; the top rock is mammillated chalcedony I found on a building site in Aylesbury in the UK. The white patina on the core is intriguing as it changes colour with the weather. When it is humid it becomes bluer, but it is white when the weather is dry. I have not been able to learn the chemistry behind this. The lower stone is banded agate, which is also a kind of chalcedony and which I bought in Sao Paulo, Brazil.