Yup, cairngorm stones are quite often found in Scottish items. Lots of times on sgians dubh, plaid brooches, etc. As I understand it, cairngorm is just a smoky-coloured quartz.

There's also quite significant use of agates in such items as kilt pins. These can look really nice, due to the plethora of different colours and the way the colours interact with each other. The STM gift shop currently has some antique kilt pins with this type stone in it.