This is Book 3 in the Sherlock Jones, Junior Detective series
Brief synopsis from back cover:
"There's a new girl at school! Lisa, the daughter of a millionaire tycoon, is beautiful, popular, and wealthy. Penny is extremely jealous and rejects Lisa's attempts to become friends. However, Penny learns that money can't buy everything. A high-speed chase which ends all hopes of finding a missing diamond causes authorities to turn to Sherlock for help. But even Sherlock's newest inventions can't help. A few interesting discoveries may be the key to solving this mystery."