Paul is making his missionary trip and he comes to the town of Philippi – not a huge town, but, due to the gold mines just outside its limits, a terribly wealthy one. And, where there’s great wealth, you will find a great many dealers in luxury goods. Enter Lydia. She was from Thyatira, a Macedonian city that was the center of the trade in purple dye and purple goods. There were a few ways to get purple dye in the ancient world, but the most elite, desired, and impressive was from a certain species of predatory sea snails called Murex. At minimum, to dye a thin garment with a light shade of this royal purple, you would need hundreds of these snails. If you had a thick garment and wanted a much more coveted dark shade? It could take 12,000 snails to get just enough to dye the trim of your garment. Needless to say, the labor-intensive nature of this product meant there was a LOT of money to be made in selling the dye and selling purple garments and decorations.
References:
https://en.wikipedia.org/wiki/Lydia_of_Thyatira &
https://en.wikipedia.org/wiki/Thyatira &
https://en.wikipedia.org/wiki/Philippi &
https://en.wikipedia.org/wiki/Tyrian_purple & https://www.researchgate.net/publication/321241274_What_is_the_Madder_with_Lydia’s_Purple_A_Reexamination_of_the_Purpurarii_in_Thyatira_and_Philippi