See
http://www.ysartglass.com/PaulYsart/PYstones.htmHowever, these were intended for use in the Caithness jewellery introduced in 1970
see article
here note dates of who made these 'Stones'
and jewellery
here To attribute a stone to Paul Ysart would require matching of canes against known Paul Ysart canes if it is not mounted on hallmarked jewellery before 1973.