Cairngorms are crystals from the Cairngorm highlands, and can be Amethyst or Topaz. Amethyst also represents the thistle heads (and Sagittarius), but Topaz is my birthstone. lol

http://en.wikipedia.org/wiki/Cairngorms
"Smoky quartz or Smokey quartz, also known as Cairngorm or Cairngormstone is a brown to black variety of quartz caused through the natural (or artificial) irradiation of aluminium-containing rock crystal. A very dark brown to black opaque variety is known as morion." "It is used in Scottish jewellery and as a decoration on kilt pins and the handles of sgian dubhs (anglicised: skean dhu). The largest known cairngorm crystal is a 23.6 kg (52 pound) specimen kept at Braemar Castle."

I've never heard of a clan gemstone. MacIntosh doesn't have one, but personally, my Glennie roots are from Braemar and Aberdeenshire with MacIntosh being just southeast from Inverness at Loch Moy. My birthday is at the end of November, so I'm Sagittarius with Astrological connection to Amethyst with a November connection to Topaz. I wear both, but mostly amethyst.