Investors interested in stocks from the Chemical - Diversified sector have probably already heard of LyondellBasell (LYB) and PPG Industries (PPG). But which of these two stocks presents investors with the better value opportunity right now? Let's take a closer look.There are plenty of strategies for discovering value stocks, but we have found that pairing a strong Zacks Rank with an impressive grade in the Value category of our Style Scores system produces the best returns. The proven Zacks Rank emphasizes companies with positive estimate revision trends, and our Style Scores highlight stocks with specific traits.Currently, both LyondellBasell and PPG Industries are holding a Zacks Rank of # 2 (Buy). This means that both companies have witnessed positive earnings estimate revisions, so investors should feel comfortable knowing that both of these stocks have an improving earnings outlook. But this is just one piece of the puzzle for value investors.Value investors also tend to look at a number of traditional, tried-and-true figures to help them find stocks that they believe are undervalued at their current share price levels.Our Value category highlights undervalued companies by looking at a variety of key metrics, including the popular P/E ratio, as well as the P/S ratio, earnings yield, cash flow per share, and a variety of other fundamentals that have been used by value investors for years.LYB currently has a forward P/E ratio of 9.06, while PPG has a forward P/E of 19.61. We also note that LYB has a PEG ratio of 1.13. This popular metric is similar to the widely-known P/E ratio, with the difference being that the PEG ratio also takes into account the company's expected earnings growth rate. PPG currently has a PEG ratio of 1.21.Another notable valuation metric for LYB is its P/B ratio of 4.43. The P/B is a method of comparing a stock's market value to its book value, which is defined as total assets minus total liabilities. By comparison, PPG has a P/B of 6.01.These are just a few of the metrics contributing to LYB's Value grade of A and PPG's Value grade of C.Both LYB and PPG are impressive stocks with solid earnings outlooks, but based on these valuation figures, we feel that LYB is the superior value option right now.Want the latest recommendations from Zacks Investment Research? Today, you can download 7 Best Stocks for the Next 30 Days. Click to get this free report LyondellBasell Industries N.V. (LYB): Free Stock Analysis Report PPG Industries, Inc. (PPG): Free Stock Analysis Report To read this article on Zacks.com click here. Zacks Investment Research