Cornwall has several tartans associated with it:

http://www.cornish-tartans.co.uk/

They were designed in the 20th century as a way to display Cornwall's Celtic heritage -- I particularly like the Cornish Hunting tartan. The Black & White which is found in almost all of them is symbolic of the St. Piran's Cross. St. Piran was the patron saint of Cornish tin miners.

Many Cornish miners immigrated to the US, btw, and brought the legend of the "Tommyknockers" with them! ;)

Cheers,

Todd