Some Gold Nugget Vdi Numbers

[Steve Herschbach]

Here is a photo with some gold nuggets from Alaska, Australia, and California that I tested recently to show how VDI (visual display indicator) numbers vary dramatically with size, shape, and purity.

Metal detectors do not know what metal is being detected. The target id number is based first on the conductivity of the item and also by the size of the item. Low conductive targets have low numbers, but the larger they are the higher the numbers go. Silver is the best conductor with gold being moderately conductive by comparison. Small gold items read very low, in the foil range, but the larger the nugget, the higher the numbers will go.

Oddly enough adding silver to gold causes the conductivity to drop dramatically instead of adding to it. Pure metals are far better conductors than alloys. That being the case the Alaska gold has much lower conductivity than the Aussie or California gold.

I have always used a U.S. nickel as a surrogate for about a 1/4 ounce gold nugget, a flattened nugget of that weight being close to a nickel in physical size. Part of this little study is to show how close to reality or not that may be, or any test items like lead or aluminum.

I do not have much in the way of “normal” metal detectors these days. The closest I have right now is a White’s DFX which reads a nickel as 22 VDI, dime 78, and quarter 84. The White's VDI range is close to being a standard, with negative numbers relating to ground minerals and ferrous items, positive number non-ferrous. The range is from -95 to +95 with non-ferrous items falling between 1 and 95.

The photo shows tests I just did on a variety of gold nuggets from Alaska, Australia, and California. The Australia gold is the purest, probably around 95% or better. The California is around 90% plus. The Alaska gold is much lower purity, closer to 80 – 85% average. You can see the purity differences in the color - pure gold is a very rich gold color, less pure gold much paler in appearance.

Click for larger version....

Gold nugget target id numbers

A few things become immediately obvious. Larger size means higher VDI numbers. However, purity appears to be even more important.

Shape, thickness, and solidity all matter – skin effects? Smooth solid masses read much higher than nuggets with pitted surfaces.

All weights are in grams except a couple larger nuggets which are Troy ounces (ozt). There are 15.43 grains to a gram. 31.103 grams per Troy ounce.

In general in all three locales you can say that nuggets under 2 grams are going to read in the foil range.

As nugget size increases however huge disparities are obvious due to purity, with all but the largest Alaska gold reading at much lower VDI ranges, and Australia gold very high numbers.

There are some odd ones that prove the situation. The Alaska 29.82 gram nugget is just under one ounce, but VDI 21, almost an exact nickel reading. This is because this nugget is probably 75% - 80% gold. You can see the color difference compared to the Australian gold next to it. It also is deeply pitted. The 4.93 gram nugget directly under it is solid and smooth and about 85% pure and so has a VDI number double what you see in the much larger nugget.

The Alaska 15.19 gram is round and solid but has quartz mixed with it, maybe 80% gold in metallic portion, only 12 VDI. The 1.25 oz Alaska in lower right has a lot of quartz and metallic portion is maybe 75% gold, so only 28 VDI.

But get big enough, and at 6.52 ounces, 85% gold, solid and smooth, you get a reading up in half dollar 90 VDI range.

For California gold I am guessing that at about 3 grams you get a nickel reading but in Australia it might be closer to 1.5 grams, and in Alaska closer to ½ ounce.

Bottom line? The nugget size to get a U.S. nickel reading is all over the map from roughly 3 grams to 15 grams but can go up to nearly an ounce for nuggets of low purity with included quartz and pitted surfaces. Saying a U.S. nickel is roughly equivalent to a 1/4 oz nugget can be true and is probably as close as you will get to some sort of average, but reality is the range of nuggets that have a VDI the same as a nickel is pretty surprising.

The final zinger is that these are air tests. Ground minerals will change the numbers, typically pulling them down. The worse the ground mineralization, the lower the numbers will shift.

 

There are a few lessons here. The first being that if you know nothing about the gold you are chasing you need to dig all targets or at a minimum all non-ferrous targets. However, if you do have a target id detector and get to know the gold in your location well, you can cherry pick with some degree of accuracy. The number one factor really is size because large nuggets are very rare. Certain areas despite wishful thinking simply do not produce large gold. If you know for a fact all the gold ever found in an area is in small gram size nuggets and even smaller, you can figure high VDI numbers are probably shell casings or some other undesired target. Further, in places like Alaska with low purity gold (not all of it - Alaska is a big place) then low VDI numbers will be the norm. The numbers speak for themselves however and you can draw whatever conclusions you want.

I have to admit that while I know all this intellectually from years of detecting to see it laid out clearly in a simple photo really drives the lesson home. It took rounding up some Australia gold and California gold to really make it a good comparison.

To further illustrate that gold as a rough rule boils down to "the larger the gold, the higher the target id number" here is the standard White's scale as printed on several top end metal detector control boxes. It shows where gold coins, gold rings, and gold nuggets generally fall on the White's -95 to +95 scale where negative numbers are normally ferrous. Pay extreme attention to the fact that White's says small gold can fall as low as -20 on their scale - deep into the ferrous range. At the other extreme a $20 gold coin may read as high as a silver dime or quarter. The gold range covers the majority of the metal detector target id scale.

White's Electronics standard target id scale -95 to +95

[fredmason]

That was a lot of work!

Still looks like you can't be sure until you dig it...

I presume the DFX is the detector used?  Maybe you could do that again with the CTX...just kidding

fred

[Reno Chris]

I wonder what your nice specimen from last summer would show?  A foil VDI is what I would expect. 

[Swifty]

• When using my ctx I mostly go by vdi numbers. Nugget shooting with a goldbug pro it's sound, and my gpz is a dig everything machine.

[beatup]

a great little tutorial Steve.

[gambler]

steve, send me those nuggets and I'll test my mx sport.   maybe

[strick]

17 hours ago, Reno Chris said:

I wonder what your nice specimen from last summer would show?  A foil VDI is what I would expect. 

Yea been waiting a while to see that one...pretty please?

strick

[FishBamboo]

Stick, I noticed in your interests you like Water Boarding. Did you work for the CIA? I know sorry off topic but could not resist.

Steve, Great comparison. Thanks for the time you put into all of this. 

[strick]

Great chart Steve.  I have a 18k ring that rings higher then my other gold rings and I always wondered if it was because of the purity. I'm gonna test it today to make sure it's not a fake.  I've read that if I keep digging zinc penny numbers with the ctx (12.35 range) I might eventually get a big gold ring. It has not happened yet. Just for fun I ran my biggest gold nugget over the ctx and it did ring way up there.

Fish- classified info...maybe around the campfire one day.