Taurus is the sign of the Zodiac ruled by Venus. The most commonly worn Taurus gemstones are diamond (or white sapphire), emerald, and blue sapphire.
Diamond is the gemstone for Venus and rules over the Taurus 1st house. Most astrologers recommend diamond for Taurus ascendants for this reason. However, years of research at Astrological Gem has determined that one should not blindly recommend diamonds as the default Taurus gemstone. Taurus is an earth sign, and Diamond appears to heavily activate the air sign of Libra, which falls under the Taurus 6th house (conflict). If it is to be worn, it is best paired with something like a blue sapphire for a deep green emerald for grounding. It can be somewhat physiologically overwhelming if too strong of a diamond is worn. When it is compatible for the Taurus physiology, it promotes brightness and resilience.