Rubies are gifts for milestone anniversaries, including the 15th and  40th anniversaries. Husbands, keep this in mind if you have a big  anniversary coming up with your wife!

The largest known ruby in existence is the Liberty Bell Ruby, which was found in the 1950's in East Africa.

It weighs over four pounds and was sculpted in the shape of the Liberty Bell.

The Liberty Bell Ruby was stolen in a jewelry heist in 2011, and four  suspects were arrested for the theft in 2014. Nonetheless, police have  been unable to find the stone since the heist.

The National Museum of Natural History is home to some of the most famous rubies in the world.

One of them is a 23-carat Burmese ruby that was donated to the museum by  philanthropist Peter Buck. The stone came from Mogok, a region in  Myanmar, in the 1930s.

The most expensive ruby ever sold was the Sunrise Ruby, which was purchased by an anonymous bidder in 2015 for $30 million.

It is the most expensive colored gemstone ever sold, and the most  expensive gemstone overall other than famous diamonds, such as the Pink  Star Diamond that sold in April 2017 for $71.2 million in Hong Kong.

One of the rarest types of rubies is the Star Ruby. The surface of the  stone has a six or twelve rayed star on it, created by tiny needle-like  inclusions in ruby.

These inclusions give the stone a silky sheen after it's been polished, and it allows the ruby to shimmer as it moves in light.

Star rubies come in the same shades as traditional rubies, but pigeon's blood star rubies are the most sought after.