Translated by Ralph Anzarouth and an anonymous friend
Jealousy is the opposite and the antithesis of love and peace and it takes two forms: the first is to feel bad about the good fortune of others and jealousy at one's fellow man's success. The second is rejoicing at the misfortunes of others. Both are very negative attitudes unless when they follow the examples of our prophets, peace be upon them, who regretted and wondered why the evil succeed or rejoiced at the vengeance of the glory of Hashem upon His enemies.
But one can find no worst trait than when this desire sprouts in somebody's heart solely out of malevolence and wickedness. And our early Sages have already said that the envious person is worst than the covetous one. In fact, when a king offered to each of these persons to make a request which he would fulfil, the covetous person asked that he should be given double what his companion would receive. And the envious person requested that one of his eyes should be gauged out so that the first should loose both his eyes.
We might compare the trait of envy to a species of bird called "Nibbio",1 about which it is said that it is so envious that if he sees his progeny getting fat in his nest he will bite them repeatedly with the sharp part of his beak on their ribs until their flesh will be lean instead of fat.
King Solomon in his wisdom stated (Proverbs 27, 4): "Who can withstand jealousy?". He further said (ibid, 14, 30): "A tranquil heart sustains the body and envy decays the bones". The Sages of blessed memory related (Talmud Bavli, tractate Shabbat 152b) a story about diggers and added: "The teacher who taught you the book of Ecclesiastes2 did not teach you the book of Proverbs, as it is written (ibid.): ‘Envy decays the bones’." In other words, the bones of the envious person will rot etc. It is also said (Job 5, 2): "Anger will kill the wicked and envy will slay the simpleton". The wise man said: "Love cannot coexist with envy and if one wishes to get even with the envious person he should (Psalms 34, 15) 'turn away from evil and do good'."
Jealousy will make your neighbour’s crop appear more abundant than yours. The successful person cannot be exempt from envy. Homer said that one should beware of the envy of his friends and associates more than his enemies'. This trait caused the split between the fist brothers in Creation, Cain and Abel, since when Cain noticed that (Genesis 4, 4): "And G-d turned to Abel's [offering] and did not turn neither to nor to his offering", the fire of envy burned within him (ibid. 4, 8): "And Cain rose against his brother Abel and killed him", thus becoming the first murderer, while the first blood which spilled upon the earth was pure and innocent.
Notes of the translators:
 A kite.
 In the book of Kohelet (12, 7) it is written: "The dust will return to the land in its original form".
The whole book Tzemach Tzadik in Hebrew (printed in Rashi characters) can be downloaded in PDF format and read online at Hebrewbooks.org. Chapter 7 about jealousy is found at pages 16 and 17.