Diamond — gemstone of Venus.
Diamond decreases Pitta and Vata and mildly increases Kapha. It strengthens the reproductive system, the bones and Ojas and helps promote overall strength, beauty and vitality, aiding in longevity. It is particularly good for women in developing female energy.
Diamond increases creativity and passion in general, whether in terms of art or in terms of sexuality, but gives fortitude, steadiness and resolve as well. It strengthens vision, imagination and insight.
Diamond is good for promoting devotion, Divine love and Bhakti Yoga. It also improves higher perception, opens us up to subtle energies and is good for Tantric practices, not just sexual but practices using the creative imagination and subtle perception including astrology.
When not to wear a gemstone for Venus
Gems for Venus as a benefic are relatively safe. However, they can increase sexuality, attachment and power of the ego or the senses if we are caught in the lower aspects of Venus energy. Hence we should strive towards the higher Venus energy of devotion and surrender, We should not use gems for Venus if we have a high sex drive or a strong need for comfort and luxury.
Astrological factors are Venus as a lord of malefic houses (like for Sagittarius and Pisces Ascendant), or a strongly sensual Venus (Venus in fixed signs or aspected by Mars in houses like the fifth, seventh or twelfth.