Mexico 8 Reales 1818 D Rm Weighing 25 Grams

The question about authenticity can not be answered from the weight alone in this kind of a case. A coin this light in weight would, in eras of stability, be classified as a counterfeit - period.

In the emergency periods around wars the coinage was more variable. Alloys could vary, however period assay reports provide no support for any great number of substandard issues coming out of Royalist mints. In fact just the opposite seems to be the case - most coins were made HIGH in assay value. The coin (alloy) therefore should test just over 10.3 in specific gravity regardless of the weight.

A coin with an alloy deficient in silver would be either a counterfeit or an issue made by the rebels who had no reason not to issue low grade coins. Either way the coin was a fraud, but one made for circulation during the same time frame.

This coin is a Durango issue - these coins - provided they were struck from silver that came from mines of the state of Durango open during the War MUST contain in excess of 1/2% of gold compared to the weight of silver in the alloy. All of the Durango mines show higher ranges of gold contamination than does Mexico City. I would therefore expect contamination right at the parting limit.

An XRF test in this case would likely disclose a modern forgery and make a clear divide from either a Royalist or Rebel issue.