There are two basic ways to refer to tools of the trade. One is strictly legal and the other is much more colloquial, whose meaning may have been borrowed from Old English.
The legal use
The term usually comes into play in bankruptcy and sometimes divorce proceedings. In the case of bankruptcy, the courts refer to tools of trade as those things a person needs to make a living. These can cover a wide gamut of things. For example, a carpenter would need his hammer, saws, power tools and other tools involved in his craft. They are normally exempt from attachment to creditors because they are things that are necessary for the person to make a living. The term "tools of trade" can be harder to define if it involves things like automobiles to get to and from work, particularly when matters are complicated by the location of the person in bankruptcy action. If the individual lives in a remote area, for example, and has no access to public transportation, it could be argued that the vehicle is a tool that allows the practice of a trade.
Another legal consideration
Suppose a famous musician (a violinist) files for bankruptcy and then performs, using a very expensive Stradivarius violin. Certainly, his creditors would like the item sold to satisfy some of his debt. But suppose further that the Stradivarius is a major part of the attraction for audiences to see him perform. What then? Would a highly-prized violin be considered a tool of trade or would the court order that the Stradivarius be sold to have the profits distributed among creditors, forcing the violinist to use a less satisfactory instrument?
The legal details are endless
What about a man with a singing frog, or a professor with a rare collection of books or one-of-a-kind manuscripts on which he relies for research determining the physics behind black holes? The legal ramifications and circumstances are extremely particular to the case at hand.
The colloquial use
No one is exactly sure of the etymological roots of the phrase "tools of the trade," but they seem to point back to Old English common law where the phrase was apparently first used. Since that time it has become a common expression referring to those things that a person needs to make a living. It is not strictly limited to "trades" as are conventionally thought of, like carpenter, plumber, mechanic, roofer and the like. Trade, in colloquial use, can mean anything from conductor of an orchestra to conductor of a train to a brain surgeon. It refers to whatever practice a person employs for her livelihood.
Safe to say
If you're a gemologist or a banker who has jewels or cash, the courts would not view these items as tools of the trade. The judge would see that the proceeds are distributed to the creditors.