The April Birthstone is Diamond
Nearly octahedral diamond crystal in matrix. 
Diamond in the Rough
The April birthstone is Diamond. Diamond is well known as a material with exceptional physical qualities, most of which are derived from the strong covalent bonding between its carbon atoms. Diamond also has relatively high optical dispersion which is attributed to its ability to disperse light of many different colors.
The birthstone for April is named Diamond an in ancient Greek it means “unalterable" or "unbreakable".
Diamond popularity has risen since the 19th century because of improved cutting and polishing methods and growth in the world economy.
A number of large or astonishing diamonds have gained distinction, both as superb examples of the striking nature of diamonds and because of the illustrious people who wore, bought, and sold them.
Natural Black Diamond known as "shaan-e-kolkata" with a weight of 121.32 carats 
Considered to be the hardest natural substance found on Earth, Diamonds are valued for their beauty in jewelry and their uses in modern technology. Diamonds are the only 10 on the Mohs hardness scale, making them ideal as cutting and polishing tools.
Diamonds are considered “fireproof” as they need an exceptional amount of heat (about 1292 degrees Fahrenheit) to actually burn. This hardy gemstone also has the greatest thermal conductivity causing a Diamond to always feel cool to the touch, earning Diamonds the nickname “ice".
Diamonds have also been called a girl’s best friend, as was made popular by the Marilyn Monroe version of the 1940’s song originally written for the Broadway production of Gentlemen Prefer Blondes.
The characteristics of Diamond
Diamonds occur naturally in a wide range of primary colors. The most common varieties are yellow and brown, while the rarest are blue and red. The most widely used and desired for jewelry however, continues to be the crystal clear versions.
- By Unknown USGS employee [Public domain] via Wikimedia Commons
- By Trishtha (Own work) [CC BY-SA 4.0], via Wikimedia Commons
- By Mario Sarto (Self-photographed) [GFDL or CC-BY-SA-3.0], via Wikimedia Commons
- By Hustvedt (Own work) [CC BY-SA 3.0 or GFDL], via Wikimedia Commons
- By Eurico Zimbres FGEL/UERJ (Own work) [CC BY-SA 2.0 br], via Wikimedia Commons