The state of Bill and Hillary Clinton's union is apparently strong - at least judging by the mongo diamond that Hillary was sporting on her ring finger yesterday.

Sources say the former President quietly gave the iceberg-sized bling - thought to exceed 3 carats - to his wife months ago, in advance of their 30th wedding anniversary on Oct. 11 last year.

But the sparkling stunner is so big that the former First Lady has been nervous about wearing it and hasn't broken it out of her jewelry box on a regular basis until recently.

The big bauble was on full display yesterday, when the junior senator visited University Settlement, an outreach center on the lower East Side, to talk about cuts to early childhood education proposed by President Bush.

"It was a gift from her husband," was all Clinton spokeswoman Jennifer Hanley would say.

Read more about Hillary Clinton's diamond ring at http://www.nydailynews.com.