LELAND BLUE

Leland Blue stone is not actually a stone at all — it's industrial waste glass called slag, the remnants of an iron smelting operation by the Leland Lake Superior Iron Company more than 100 years ago. 
Leland Blue was formed when molten products in the Leland Lake Superior Iron Company's furnaces were quickly cooled, creating a glass-like solid in much the same way that the volcanic glass called obsidian is made. 
Northern Michigan was known for its iron ore and mining industry, which is just how this slag has formed and created the beautiful Leland Blue Stone. 
The process, roughly described, is that during the mining of ore, it eventually gets heated to extreme temperatures and separated into iron ore and then other natural byproducts. From here, these other natural byproducts cool and mix and as a result, form slag, sometimes containing blue green colours.