After thousands of years, these shells were mineralised and formed beautiful agate.

mineralised 是动词 mineralise 的过去分词形式，表示被动含义。mineralise 由名词 mineral（矿物）加上动词后缀 -ise 构成，意思是"使矿物化"或"矿化"，指有机物质逐渐被矿物质取代的过程。