From my post of last year (thread name: Gold, Platin and Diamonds):

Platin is lower karat gold, nothing to do with platinum (if it was it would need to be a lot cheaper than it is!)
AFAIK, what follows is accurate but I'm happy to hear suggestions.
All gold shops in Thailand are required to display their buy and sell prices for gold. In Cambodia they rarely display and thus will be looking to vary the price according to the identity of the customer. That means you need to be a regular customer to get the best prices.
What they describe as 24k gold, is in my opinion, a slightly lower karat. I say this because it is not soft enough and is priced too low compared to the world price. It is, however, too soft to be used for anything other than, for example, rings and chunky bracelets and necklaces. Those bangles that the women like to wear in multiples of seven, they are always made of platin.
Platin is usually imported from Vietnam whereas the so-called 24k is made in Cambodia. Because of this, the spread between the buying and selling prices is much highter in platin. In 24k, it is about 3%, in platin it will be around 6%. This is important because most Khmers will regularly sell back their gold or exchange it for a new piece. The shops will offer a tighter spread on exchanges compared to buy-backs. And you need to go back to the same shop to get the best spreads. For this reason you should keep the receipt but they may recognise it as theirs, or have a record of the transaction.
You have to pay making charges. A simple gold band will be only $3 but a bracelet can be $20. Unscrupulous sellers may charge a lot more.
Gold in Cambodia is priced in chi and damlung. One chi weights 3.749 grams One damlung is 10 chi. You can check the current spot price on kitco.com and use a calculator to work it out. Remember that gold is priced in troy ounces (31.1 grams).
I like the shop on st.110 opposite Psar Chas. It has a TV screen displaying their current buy and sell prices for 24k and 18k (platin). They have aircon and English speaking staff. There is a big shop somewhere in Russey Keo that people say is the best but when I have compared the prices, the st 110 shop has been just as good.
I don't know about diamonds, personally I would avoid them. Gold is a superb investment as every Khmer knows. So long as it does not get lost or stolen (actually or allegedly).
Further to the above, I had a good conversation with the people in the st 110 shop.
They say they source gold from Hong Kong and Singapore in the form of 1kg 9999 bars. They then make "Khmer gold" jewelry with that pure gold and sell it as 24 karat. They also blend it with base metals to create "Khmer platin", which is in 75% and 70% gold formats.
In addition they import platin from Vietnam which is 60% gold.
Confusingly, all 3 forms of platin are described as 18 karat. Of course they sell for different prices. Yesterday the selling prices were:
24k - $211
75% - $158
70% - $146
60% - $131

The spreads were tightest on the 24k and higher on the Khmer platin and highest on the Vietnam platin.
If you want to buy gold bars, they can sell 1kg bars (yesterday's price $56,126). If you want a smaller amount they will simply cut a pice off a 1kg bar. No making charges obviously!