Can anyone tell me any further information about the above seal fob (25 mm dia. by 35 mm tall) and help me date it more precisely?

I photographed this fob, which I understand was found in North East England, some years ago when visiting the open day of a local metal detecting club.  It appears to comprise a silver mounting (although no hallmark etc. is visible) which holds a purple coloured intaglio, possibly an amethyst. The engraved design on the intaglio appears to show an early mine pump engine house together with a hay stack type boiler. Based on the style of the fob mounting and what little I know of such things I estimate that it dates from the late 1790s to 1820s.

The engine house I take it depicts a steam engine and not an atmospheric engine (allowing in this case for some artistic licence given the very small size/scale of the fob). I'm no expert on steam engines but I seem to remember there are differences in engine house style and design depending on which type of engine they housed. Also when did hay stack boilers go out of fashion in favour of Cornish or Lancashire etc. type boilers? The answer to this may again help in dating the fob although I expect this date may have varied from coalfield to coalfield?

My guess is that given the unusual subject matter chosen for the fob's intaglio (i.e. not the usual coat of arms or initials etc.) it probably belonged to a colliery owner or one of his agents who was proud to advertise his investment in the new mining technology of the time. Can anyone identify the once owner of the fob based on any existing seal impressions in libraries or archives?