Kays of Scotland is the sole provider of what item used in the Winter Olympics?

Kays Curling has been supplying the official curling stones at almost all Winter Olympic Games since Chamonix 1924 and the company mainly resources materials from Ailsa Craig, an island 16 km off the Scottish mainland. Two types of granite, Common Green and Blue Hone are mixed to create these curling stones in this almost 200-year-old process, and usually the company manufactures one stone per hour, a staggering total of 38 a week.

https://www.bbc.com/news/uk-scotland-60259628