Bright, sparkling diamonds.

These are some of the biggest tartrate crystals I have ever seen on the underside of a cork.  Not surprising, I suppose, considering the age of this Sauvignon Blanc (SB) and the fact that it has been stored upside down in a cool cellar for the past 5 years.
Tartrate crystals, also known as wine diamonds, are a harmless, flavourless by-product of winemaking.  When tartaric acid, an acid naturally found in grapes, binds with potassium (under low temperatures) potassium bitartrate (cream of tartar) is formed.  If not removed from the wine, by a process called cold stabilisation, excess tartaric acid will solidify and form crystals when wine is chilled in a refrigerator. Voila, wine diamonds!
The bejeweled-wine, in this instance, was a Kobalt Wines 2011 Sauvignon Blanc (Knights Valley AVA).  At the grand old age of 6, this is the oldest SB I have had in a quite a while. It's not that I don't like older white wines, I just tend to drink whites when they are young and fresh. One of my favourite things to do with Thud, on visits home, is share a bottle of an older vintage Sancerre (Ladoucette Comte LaFond, an absolute favourite, springs immediately to mind).  I just don't think New World white wines age as well as those from the more traditional wine regions of the world - yet. Having said that, the Kobalt SB was a delightful wine, very food friendly and very moreish.

Wine diamonds.

This is what happens in a bottle of wine if cold stabilisation has NOT taken place. It is not essential for wine to be cold stabilised, as evidenced by the underside of the cork in the photograph above (from a bottle of Chenin blanc), but it does make it more palatable to the consumer.
Crunchy wine, anyone?