J. A. Darke 
Creature in the Crystal [PDF ebook] 

As a little kid, Val was intrigued by anything to do with the supernatural. So when her cousin Ronnie comes to visit for a long weekend, with a book of spells, a Ouija board and some mysterious gems in tow, Val’s curiosity is re-ignited. But soon Ronnie’s fearless spirit and sense of spooky fun starts to haunt Val, who keeps having nightmares and witnessing a monster following her around the house. Can the girls figure out the truth before they’re forced to come face to face with a real-life haunting?