You might be Cancer as per Western Astrology but for Vedic Astrology you are Scorpio. This is called Rashi or the Moon Sign. One major difference between Western and Vedic Astrology is in Western they follow the Sun and in Indian they follow the Moon.
Yes you can wear Diamond as it is the stone of the 9th house, means stone of your destiny or the bhagyastan. You can wear Blue Sapphire also.