June - the traditional birthstone is Pearl; the alternative stone is Moonstone or Alexandrite.
The following information is taken from the Britannica Student Encyclopedia.
Pearls can be black, brown, gray, rose, red, blue, green, purple, yellow, and white. No one knows exactly how pearls develop their color, since one oyster may produce pearls of several different hues simultaneously.
And this from Bernardine Fine Art Jewelry :
Pearl is the official birthstone for the month of June as adopted by the American National Association of Jewelers in 1912. It is also the birthstone for the Sun Signs of Gemini and Cancer. See the birthstone tables for additional references to this stone. Fresh water pearls are given on the 1st wedding anniversary. Pearls are also given on the 3rd, 12th and 30th anniversaries. Fresh water pearls are given on the 1st wedding anniversary. Pearls are also given on the 3rd, 12th and 30th anniversaries.
An organic gem, pearls are formed inside mollusks such as oysters and mussels. They are formed when an irritant such as a tiny stone or bit of sand gets inside the mollusk's shell. A lustrous substance, called nacre, is secreted around the object to protect the soft internal surface of the mollusk. As layer upon layer of nacre coats the irritant, a pearl is formed. Light that is reflected from these overlapping layers produces a characteristic iridescent luster. This process of building a solid pearl can take up to seven or eight years.
The most valuable pearls are perfectly symmetrical, relatively large and naturally produced. They have a shimmering iridescence which is called orient luster. The principal oyster beds lay in the Persian Gulf, along the coasts of India and Sri Lanka, and in the Red Sea.
Moonstone - from The Book of Stones:
Moonstone is a feldspar mineral, a potassium aluminum silicate with a hardness of 6 to 6.5. It is found in Sri lanka, Madagascar and Myanmar (Burma), Australia, Idnia and the United States. It was used in Roman jewelry nearly two thousand years ago and even longer in the Orient, and was a popular jewel choice during the Art Nouveau Period. In India it has always been a sacred stone. Metaphysically, it is said to reconcile estranged lovers and to promote restful sleep.