At one point, I had to do a bit of research on sapphires themselves. I found out they come in many different colors, including pink!
And then there were the pretty people. Sometimes I like an image of my characters in my head, and I'd had these images of Lily and Darlington in my head from the very first book in the series. This is how I pictured Lily, who's a redhead and called The Countess of Charm.
But this picture of Darlington was my absolute favorite. I think the cover designers did a great job of capturing the idea I had of him in my mind. Tell me he doesn't look like a debauched Regency gentleman.
I had to do a bit of research on clothing, as I always do for each novel. At one point, Lily and Andrew go riding and I did look at a few riding habits in order to describe hers for the reader. Here's one illustration I consulted.