IF YOU'VE BEEN following the Smithsonian gem-acquisition series now running in The Post, you may have experienced some shocks. First there was the news that that large diamond you were planning to leave to the museum is quite likely to end up on the open market or stored in some dusty vault rather than to be enshrined in the display case you had in mind. Then there was the nasty disclosure that many of those philanthropists you have always admired may be in the business for more than sweet charity's sake.

Upon reflection you will realize, of course, that none of this should have come as a surprise. Museum collection-building is a business like any other and a very competitive one at that. Aggressive collectors will naturally stockpile unwanted gems and art objects as potential barter items against the truly rare piece--especially when the cost of those stockpiles to the museum is apparently nothing.

The awkward side of this business arises from the fact that the stockpiles do, in fact, cost money, money that is lost to the Treasury when donations are claimed as deductions against taxes otherwise owed. The charitable deduction was, of course, intended to encourage giving. The objectionable aspect arises because, under current rules, the Treasury--which is to say other taxpayers--ends up paying a very high cost for items of questionable value because of the way that the donation-market operates.

When a high-bracket taxpayer wants to lighten his tax load, what better way than to do it by donating some object--be it manuscript, jewel or painting --whose value is at least arguable? It is not difficult, as the Smithsonian series demonstrates, to find an appraiser who is willing to take the most generous view possible of the object's value--after all, why should he be stingy? And the museum doesn't care what value is claimed since it won't count against its budget, and the bigger the going tax break, the greater the incentive to other potential donors. In short, a situation exists in which all parties to the deal have an incentive to inflate the value of donations--except the Treasury, which must ultimately pay the cost.

The IRS has been cracking down on some of the more abusive uses of the donation shelter. This, however, is a very expensive and intrusive procedure for everyone involved. A much better system would be to eliminate deductions for items altogether. This wouldn't discourage legitimate giving. The donor would simply have to sell the item on the open market--thereby establishing a tangible value --and donate the net proceeds to the museum. The museum, in turn, could use the money to buy the object--if it really wanted it. Some money would be lost to middlemen in the process, but that would be a small price to pay for a measure of discipline in the charity market.