If lustre sounds subjective to you (it really isn't, it just takes a lot of experience to evaluate it) just wait until you learn about eye-appeal. Eye-appeal is, by far, the most subjective aspect of grading.
To give you an idea about the importance of eye-appeal, I will now tell you my favorite rare coin war story. In late 1979, Jerry Cohen, then a partner in the Abner Kreisberg Corporation, held a coin auction in Los Angeles. It was an especially beautiful sale with many interesting coins. One of the most interesting (and rare) coins in that sale was a 1795 Small Eagle bust dollar, described simply as Uncirculated. It was a gem coin with superb surfaces, lustre and strike. Unfortunately, the toning was positively hideous. The coin simply lacked eye-appeal.
The Coin Dealer Newsletter listed "bid" that week at $22,500 in MS-65, and most of the top dealers (myself included) were willing to bid somewhere in the $20,000-$25,000 range for the coin. Steve Ivy, my good friend, but also at that time my arch rival (we're now business partners - how the world changes!) bought the coin for $28,000.
I expected him to "dip" the coin to remove the toning, but business was so brisk at that time that he never got around to it. The coin just sat in inventory for the three or four most explosive months in the history of the coin market. Nobody would buy the coin from him because it lacked eye-appeal. "Bid" doubled to $45,000 by February, 1980. After intensive negotiations (lasting less than a minute, I think) I managed to buy that turkey of a coin from Steve for $33,500. In my mind, this was tantamount to a loss for Steve: He had purchased the coin for 25% over bid, and resold it to me at 25% below bid. (It is worth noting that grading standards throughout the industry had certainly not tightened over that time period.)
I suppose I had some reservations about dipping the coin. Even for an expert, dipping a coin is a risky undertaking. (And I would never recommend that a novice ever dip a coin). What if the toning hid some unpardonable flaw? Or what if the lustre became dull as a result of the dipping? Still; no guts, no glory!
A quick dip in Jewel Luster produced the most stunning, blazing white semi-prooflike gem early U.S. silver coin I had ever seen! Really, nothing had changed except the eye-appeal factor. The coin was transformed from a "technical MS-65" with no eye-appeal, to a wonder coin, a coin that had it all. The coin that Steve couldn't sell now had suitors waiting in line. There were literally half a dozen knowledgeable buyers begging me to give them first shot at the coin. I sold it to a dealer in the Boston area for $137,500.
Since eye-appeal can be so important, it is critical that you develop a sense for it an art critic's eye for the aesthetically pleasing. My best friend, Marc Emory probably has the best eye for quality in the coin business today. Marc taught me almost everything I know about eye-appeal. He believes, as I do, that eye appeal can best be divided into three distinct areas: Toning, balance, and that certain inexplicable: aesthetic attractiveness.
First, toning. Here are photographs of five coins, all approximately equal in surface preservation, strike and lustre. They are arranged from worst to best in terms of toning.
Note that coin #4 is approximately equal in eye-appeal grade to a coin with no toning at all. However, a flawless gem with no toning and uniform texture on both sides can still be given a 5 for eye-appeal.
The best way to grade a coin for eye-appeal is to make your own judgement as to how your coin's toning compares aesthetically to the coins photographed above. Assign a grade, using fractions where appropriate. Give it a 4, if the coin has no toning at all. Then we'll make adjustments for balance and aesthetic attractiveness.
Balance can best be defined as the coin's overall consistency. Does the toning on the reverse match (or at least go nicely with) the toning on the obverse? Is the texture of the surfaces roughly equivalent on both sides, or is the obverse frosty while the reverse is prooflike (for example)? Does the coin have pleasing balance, or is it somewhat lopsided?
A good general rule is to add up to a point to the toning score for perfect balance. (Of course, the maximum score is always 5, so don't add anything if you already gave the toning a 5). Or, you can subtract anywhere from 0 to 50% of the toning score for lack of balance, depending on severity. (In no case should the total score be less than 1).
Finally, to the number above, you can make a final adjustment for that indefinable aesthetic attractiveness. Try this little trick I used to use in college to decide which subject to study next. Say I had a choice of studying for a Sociology exam or writing an Economics paper. I would flip a coin: heads for Sociology, tails for Economics. But instead of actually looking to see if the coin would come up heads or tails, I'd try to notice which result I was subconsciously rooting for. If I found myself hoping that tails would come up, I wouldn't even look at the coin. I'd just start writing the Economics paper.
You can do the same thing when you grade eye-appeal. Consider the eye-appeal grade you gave your coin based on toning and balance. Now look at the coin and try to notice exactly how high or low your subconscious thinks that number is. That difference is purely aesthetic, quite subjective, and probably impossible to explain. Yet it's hard to deny its existence, as you will most likely notice it time and time again during the grading process. just follow your heart, and adjust your final number as you see fit, just so you keep it between 1 and 5.
Eye-appeal is the grading factor about which expert dealers will most often disagree. You know what you like, and I know what I like. Once you have looked at a few thousand coins, your opinion about the eye-appeal grade of a coin will be as valid as mine or anyone else's.