I had a look at Shirley Bury, Vol 1 of Jewellery, The International Era, 1789 -1910 for amethysts.
“Stones such as amethysts, in high fashion in the late eighteenth and early ninteenth centuries, were expensive on account of the relative sparseness of the traditional supplies from India and elsewhere. Once a prolific source of the stone was found in Minas Gerais prices fell and amethysts slipped down the social scale.”
The period of high fashion for amethysts fits well with Queen Charlotte (1744-1818). Minas Gerais is a state of Brazil.