Why do gold prices differ by country?
World spot price (USD/oz) is the same everywhere. What differs by country is the local UNIT (tola in India, lượng in Vietnam, tael in HK), the local currency (FX), import duty, VAT, and dealer premium. This page shows the raw spot×unit math; per-country retail prices are at /<country>/prices.
Which weight is best to compare across countries?
Compare in grams. It's the only unit every market understands. Once you've got the per-gram USD rate, multiply by FX and add local premium (5–25%) to get what you'll actually pay.
What's the difference between silver, platinum, palladium prices?
Silver is ~80× cheaper than gold per oz but moves with industrial demand (solar, electronics). Platinum and palladium are rarer than gold and driven by car-catalyst demand. Palladium has spiked above gold in past cycles; long-term gold is the steadier hold.