This is from Fr. Lasance's "My Prayer Book" (published early 1900s) around p. 164, "Sanctify The Month": https://archive.org/details/MyPrayerBookHappinessInGoodness
Each month has a gemstone that represents something and reflection related to it, monthly devotion, and particular virtue.
April is devoted to the Holy Ghost (and also the Passion, as relates to Lent).
The virtue for April is patience.
The gem of the month is a diamond, which represents purity and fortitude.
April - Diamond - Purity and Fortitude - Strength of Character
"Perform your actions with a pure intention, not out of human respect, but for the love of God. Do what is right because it is right. Have the moral courage to stand up for your convictions. If you are conscientious, you cannot help but have some character."