"Silex" refers to a very specific and prized soil type in the area of Sancerre. It literally means "flint" or "gun flint" and imparts a distinctive smoky note to the wines. That is in addition to beautiful layers of citrus, green grass, and minerals. This is a classic.
There is precious little of this to go around and it's very much worth a little splurge. The days of $25 Sancerre (especially grown in silex) are sadly a thing of the past, but I could drink this every single day and be happy.
100% Sauvignon Blanc. Serve cold.