What are the special properties of the Belt of Deltora's gems?

Taken from the series' fictional book The Belt of Deltora:

DiamondThe Diamond, symbol of innocence, purity, and strength. Diamonds gives courage and strength, protection from pestilence and helps the cause of true love. But take heed of this warning: Diamonds gained by treachery or violence, or desired out of envy or greed, are ill omens, and bring bad fortune. Great evil comes upon those who gain them without honor.

EmeraldThe Emerald, symbol of honor, dulls in the presence of evil, and when a vow is broken. It is a remedy for sores and ulcers, and an antidote to poison.

Lapis LazuliThe Lapis Lazuli, the heavenly stone, is a powerful talisman which protects its wearer from evil.

TopazThe Topaz, symbol of faithfulness, is a powerful gem, and its strength increases as the moon grows full. The Topaz protects its wearer from the terrors of the night. It has the power to open doors to the into the spirit world. It stengthens and clears the mind, enabling the wearer to see through pretence and evil magic.

OpalThe Opal, symbol of hope, shines with all colors of the rainbow. It has the power to give glimpses of the future, and to aid those with weak sight. The Opal has a special relationship with the Lapis-Lazuli.

RubyThe great ruby, symbol of happiness, red as blood, grows pale in the presence of evil, or when misfortune threatens its wearer. It wards off evil spirits, and is an antidote to snake venom.

AmethystThe great Amethyst, symbol of truth, calms and soothes. It changes color in the presence of illness, loses color near poisoned food or drink, and guides the wearer towards sincerity, security, and peace of mind.