When asked if the weight and the purity of gold coin dollars have an equal part in determining their value would you be able to answer this? Most people would not because the required knowledge is usually gained through daily analysis of the gold market.
Gold bars are one thing, but researching gold coins has to be done more carefully because they are so much more prevalent in the public sector. Furthermore the variety of denominations can become very confusing. For instance the 20 dollar gold coin is going to weigh in with a value far beyond a mere 20 dollars. The same can be said for a five dollar gold coin. It is the weight as well as the purity that will help you become familiar with what the value of the coins may be.
There is a need to have a general idea of what these precious metals are currently trading for, yet the market in coins fluctuates at a slower pace than the gold bars themselves. The reason for this is the same as mentioned before. The general public does not use these denominations as the more commonplace uses will include paper money which is easier to carry.
The change to this occurred over 100 years ago and most people would have little ability to carry around large amounts of coins because they are heavy. The pure gold will be considerably more likely to be found in bar form though and it will probably never see public circulation either.