The League of Gentlewomen Witches By India Holton

Charlotte Pettifer belongs to a secret society skilled in witchcraft.
When rumours of the Amulet of Black Beryl start to circulate, Charlotte is determined to find the jewel before it falls into the wrong hands.
Which is what happens when the evil Lady Armitage reaches it first.
Demanding the help of rakish pirate, Alex O'Riley, Charlotte sets off to find the jewel.
There's just one problem: pirates and witches are sworn enemies.
But little do they know, sparks are about to fly . . .