Finished the 5th installment in the Throne of Glass series, Empire of Storms. Now, if you'll excuse me, I'll be weeping until the 6th comes out. But, seriously. Best book of the series. I do recommend re-reading (or reading, period, if you haven't yet) the prequel Assassins Blade. This book references that one quite a bit.
Obsidian Blade Necklace – "This is Obsidian–volcanic glass. The edge is wicked sharp and will cut through anything," he explained quickly. "It's the only thing on this planet, besides us, that can kill the Arum. This is their kryptonite." (The Lux series by Jennifer L. Armentrout)