I understand Jewish custom dictates that the body is to be buried with earth to speed the return of the soul to nature or to God. If this is true, why was Christ, a Jew, placed in a rock tomb and not buried in the earth? Please explain the purpose of the Jewish custom too.

The Jews did believe in burial as opposed to burning. Burning was considered a desecration of the body and was reserved for criminals either while alive (Genesis 38:24And it came to pass about three months after, that it was told Judah, saying, Tamar thy daughter in law hath played the harlot; and also, behold, she is with child by whoredom. And Judah said, Bring her forth, and let her be burnt.
See All...; Leviticus 20:14And if a man take a wife and her mother, it is wickedness: they shall be burnt with fire, both he and they; that there be no wickedness among you.
See All...; 21:9
See All...) or after death (Joshua 7:15And it shall be, that he that is taken with the accursed thing shall be burnt with fire, he and all that he hath: because he hath transgressed the covenant of the LORD, and because he hath wrought folly in Israel.
See All..., 25). The burning of bodily remains was  considered an outrage, as when Moab "burned the bones of the king of Edom into lime" (Amos 2:1Thus saith the LORD; For three transgressions of Moab, and for four, I will not turn away the punishment thereof; because he burned the bones of the king of Edom into lime:
See All...). Another reason for burial was to fulfill the judgment on man because of Adam's fall: "for dust thou art, and unto dust shalt thou return" (Genesis 3:19In the sweat of thy face shalt thou eat bread, till thou return unto the ground; for out of it wast thou taken: for dust thou art, and unto dust shalt thou return.
See All...). Burial was considered the proper way to fulfill this judgment.

However, burial did not necessarily refer to the digging of a hole in the ground and the placing of the body into the dirt as we normally do today. That was probably done at times. However, the Bible record (both in Old and New Testaments) speaks most of burial in tombs. These tombs could be natural caves (as in the cave of Machpelah where Abraham buried his family - Genesis 23:9That he may give me the cave of Machpelah, which he hath, which is in the end of his field; for as much money as it is worth he shall give it me for a possession of a buryingplace amongst you.
See All..., 19), or hewn chambers (as the place where Jesus was buried; called a "new tomb" - Matthew 27:60And laid it in his own new tomb, which he had hewn out in the rock: and he rolled a great stone to the door of the sepulchre, and departed.
See All...), or even an erected mausoleum. Since the body is placed in the earth, this is considered burial. Genesis 23:19And after this, Abraham buried Sarah his wife in the cave of the field of Machpelah before Mamre: the same is Hebron in the land of Canaan.
See All... records, "Abraham buried Sarah his wife in the cave of the field of Machpelah before Mamre." The Bible clearly calls the placing of a body in a cave a burial.

I was not familiar with your stated purpose for burial; that is, to speed the return of the soul to God. However, I found a quote that might explain this concept. Rabbi Ben Kaphra, who lived in the centuries after Christ, is quoted as saying: "For three days the spirit hovers about the tomb, if perchance it may return to the body. But, when it sees the fashion of the countenance changed, it retires and abandons the body." I would speculate that a burial in earth might more hastily convince the spirit that the body needs to be deserted. But that is only my speculation. In ancient times and in the warmth of Bible lands, the burial would of necessity take place within 24 hours. This practice was followed in other lands for centuries but is not practiced everywhere now. That practice might also be suggested by a desire to encourage the spirit's return to God. However, most of these practices are later traditions and would not affect the practices of Bible times.