Hi Tom,

Back when I made jewellery & needed a bezel for oddball shaped stones, I'd take a photo of the stone with a ruler beside it.
I'd take the photo as a "top view" then crop the image to just include the stone & whatever amount of the ruler would fit.
In MoI's top view I'd create a line of 10mm then import image & keep scaling it until 10mm on the ruler matched my 10mm line.
From there on you could create what you needed to without being concerned of dimensions being off.

And remember when first importing your image to click the "Properties" button & tick the "Embed image data in 3DM file".

HTH, Colin