Target ID / Vdi Numbers For Gold Nuggets And Gold Jewelry

[Steve Herschbach]

A common misperception among those new to metal detecting is that metal detectors can identify one metal from another. How much we wish that were true. The reality is that for all practical purposes the common metal detector target id scale is based on a combination of the conductive or ferrous properties of the item multiplied by the size and shape of the item.

There are two common terms in use for this scale. The Target ID or TID scale is the most generic. White's also popularized the use of Visual Discrimination Indicator or VDI numbers. You will see references to both TID and VDI numbers and both refer to the same thing. The problem when you use Google is that TID also refers to Terminal ID number, which is for credit card machines. VDI gets far better results as the preferred term and so is what I will use from now on.

The VDI scale is almost always arranged the same way by common convention although in theory it can be rearranged any way you want. The common scale has ferrous items on the low end and non-ferrous items on the high end. Ferrous items are like mirror images of non-ferrous items and so the most common arrangement of the VDI scale is with small items in the middle with ferrous getting larger in one direction and non-ferrous getting larger in the other direction. The ferrous and non-ferrous ranges actually overlap in the middle.

Large Non-Ferrous
Medium Non-Ferrous
Small Non-Ferrous
Tiny Ferrous/Non-Ferrous Overlap
Small Ferrous
Medium Ferrous
Large Ferrous

We can assign a numeric range to this basic VDI scale any way we want. Many early machines went with a 0 - 100 scale, with the ferrous compressed into the low end of the scale:

100 Large Non-Ferrous
50 Medium Non-Ferrous
20 Small Non-Ferrous
5 Tiny Ferrous/Non-Ferrous Overlap
3 Small Ferrous
1 Medium Ferrous
0 Large Ferrous

The idea of ferrous as negative numbers made sense due to the mirror imaging in size between ferrous and non-ferrous. A very common White's scale runs from -95 to 0 to +95

95 Large Non-Ferrous
50 Medium Non-Ferrous
15 Small Non-Ferrous
0 Tiny Ferrous/Non-Ferrous Overlap
-15 Small Ferrous
- 20 Medium Ferrous
- 40 Large Ferrous

The "positive only" 0 - 100 VDI scale seems most popular these days with other manufacturers, but the scheme varies. Two very common setups are 0-40 ferrous and 41-99 non-ferrous OR 0-10 ferrous and 11-99 non-ferrous. But as I noted you can set this up any way you want and so other scales do exist.

When we look at just the non-ferrous part of the scale, what is important is how the detector "sees" the target. In very simple terms conductive targets are either very weak or very strong or somewhere in between. Small items are weak targets. Low conductive metals are weak targets. Large items are strong targets. High conductive metals are strong targets. The shape matters. Irregular shapes  or thin items are weak targets. Rounded and thick items are strong targets.

On a conductive scale of 0 to 100:

0 = very small targets 100 = very large targets

0 = very thin targets 100 = very thick targets

0 = very low conductive metals 100 = very high conductive metals

0 = very irregular shaped targets 100 = very rounded targets, especially is a hole in the middle

Add this all up and small gold items are low on the VDI scale and large gold items high on the scale. Silver being a better conductor than gold, a silver item will read higher on the scale than the identical size and shape gold item. In general silver will read higher than gold. However, a very large gold item can read higher than a very small silver item. Chasing thin hammered silver coins in the U.K., especially the cut varieties, is not that different than hunting gold nuggets.

What you rapidly figure out is the metal detector VDI scale can only get repeatable results on certain man made items that are the same every time, like a U.S. nickel or a U.S. dime. And even these signals degrade when deep in the ground or in proximity to other items under the search coil at the same time. Given all the limitations, it is a wonder we get any degree of accuracy at all with detector discrimination systems.

With that, I give you a standardized White's VDI scale taken directly from the control box of my White's DFX. This -95 to 0 to +95 scale is common on many modern White's detectors. Nearly all other detectors have the same relative positioning of items just with different numeric scales, an exception of note being the Fisher CZ detectors, which use a rearranged scale. This DFX scale is helpful because it includes gold coins.

The main thing I want you to focus on here is the relative positioning of items on the scale. As a detectorist operating in the United States, I always pay attention to just three things 1. where do the ferrous numbers start? 2. where does a U.S. nickel read? and 3. where does a U.S. dime read? If I know those three things, I can adjust almost instantly to any detector scale in existence, because I know how everything else reads in relation to those three points on the scale.

Standard White's VDI scale

Looking at the scale you can use gold coins as a rough guide to where large gold nuggets will read, although coins being pure gold and round will read much better than gold nuggets of the same size. It might take a one pound gold nugget to read the same as a one ounce $20 gold coin, which in turn reads very close to the U.S. silver quarter reading.

On the other end, tiny gold, tiny ferrous, and salt water, being a low conductive target, all overlap. This is why if you tune out salt water on the beach, you also tune out single post gold ear rings and thin gold chains, which read like small gold nuggets. If a prospector tunes out salt alkali readings on a salt lake, there go the small gold readings. And the chart shows that if you get too aggressive in rejecting all ferrous items, good items can be lost also.

When I say small it is important to note what we are really talking about is small/weak readings. A large gold item buried very deep in mineralized ground will have a very weak reading and appear as a small target to the detector. This means a very deep large items can appear just like a very small gold item and be lost for the very same reasons as those small items. Again, think weak targets and strong targets to get a better feel for how things react in the field.

To sum up, gold and platinum are low conductive metals, and when also small in size read very low on the VDI scale, even dipping into the ferrous range. The foil range is the sweet spot for ear rings, thin gold chains, small womens rings, and platinum items. In general women's gold rings will read below a U.S. nickel and men's gold rings will fall above a U.S. nickel on the VDI scale. Nearly all gold nuggets found by most people are going to read nickel and lower just because nearly all gold nuggets are small. However, as this photo I made using my DFX and some gold nuggets shows, gold nuggets can read all over the place due to their shape and purity. Surprisingly, if you add silver to gold the conductivity drops as alloys are less conductive than pure metals. This makes many gold jewelry items and gold nuggets far harder to detect than would be the case were they pure gold. See this article for details on this nugget photo Some Gold Nugget VDI Numbers

Target id numbers for naturally occurring gold nuggets

You can get some great spreadsheets for jewelry VDI numbers for White's and Minelab detectors here.

There are no doubt many people who have read this who are just shaking their head and thinking "this is why I just dig everything". I absolutely agree, when at all possible, that is the best solution. Unfortunately it simply is not possible in some locations where trash targets outnumber the good by thousands to one. This is where knowing the VDI scale and how it works can pay off.

The best book ever written on the subject of discrimination is "Taking A Closer Look At Metal Detector Discrimination" by Robert C. Brockett. It is out of print but if you find a copy grab it, assuming the topic interests you.

[~LARGO~]

Thank you, Steve Herschbach, for this scholarly dissertation...

THIS is why I bow towards Reno each time I turn on my detectors!

~LARGO~

[Jakepilot86]

I went to a beach on Monday that was so littered with garbage that I spent hours digging the same few types of items with a nickel or dime thrown in. With the exception of learning how all the valuable items sounded (since I didn't find any of those), I can attest to those readings. The nickels were reading just like the whole pull tabs, the dimes like small pieces or the old-style aluminum bits. I dug them all to get more familiarity with my GMT, and had to have the sensitivity dialed way down. It's just so sensitive to even the small bits of aluminum. Still haven't bothered to empty the pouch of all the junk...

[Steve Herschbach]

The White's -95 to 0 to +95 VDI scale as far as I know was first explained well in the XLT Engineering Report by Mark Rowan. Here is an excerpt below. Note that extreme low VDI numbers are actually ground readings, and the extreme low VDI range is where your ground balance setting resides. Ground balance is just a type of discrimination. This VDI scheme has been used by White's since on the DFX, V3i, VX3, MXT, etc.

"For many years, White's has built detectors which identify targets based on a V.D.I. number (Visual Discrimination Indicator) which characterizes metallic objects according to their size, shape, and composition. The V.D.I. scale on the Spectrum XLT runs from -95 to +95. Large positive numbers typically indicate objects that are good electrical conductors; for example, silver dollars will come in at 92. Smaller positive numbers usually indicate objects which, because of their size, shape, or composition, are not as conductive; nickels will read about 20, and aluminum foil may come in near 5. Large negative numbers are typical of targets which are readily magnetized, but which conduct electricity poorly or not at all. Some sands or soils which have a high concentration of ferromagnetic minerals may read -93. Metals containing iron have both magnetic and conductive properties, which causes them to spread over a wide area of the scale, although most typically iron objects will fall in the range -30 to -75. (See Phase Chart).

The V.D.I. reading is an excellent way to determine the identity of most commonly occurring targets, although I might mention in passing that the only 100% reliable discriminator is called a shovel. However, as a famous metal detector engineer once said, "Life is grossly unfair" (actually, there is no such thing as a famous metal detector engineer, and life really is fair, it just doesn't want anybody to know). For one thing, the signal which a detector receives back from even moderately mineralized ground is typically much stronger than the signal it receives from the targets buried in it. This makes determining an accurate V.D.I. number for a target at any substantial depth a very challenging business indeed.

Furthermore, some targets will cause an abrupt change in V.D.I. response during the course of a single pass under the loop; the most notorious of these are the dreaded bottlecap and the dreaded small piece of foil near the surface in bad ground."

[Steve Herschbach]

Here is an example of how Nokta and Makro use the same spread of items but change the numeric scale to run all positive from 0 to 100. Note again how the low end of the range is where ground readings reside, and how ferrous, salt, and ground all overlap to a degree. Garrett and First Texas employ similar VDI scales.

Due to a situation referred to as "ferrous wrap" some hot rocks and certain ferrous items can come in on the high end of the scale. This happens with White's and other brands also.

Note that the Nokta FORS CoRe, FORS Gold, etc, and original Makro Racer use 0-40 as ground/ferrous. First Texas does the same with the Teknetics T2 and Fisher Gold Bug Pro, etc.

The new Makro 2 compresses the ground/ferrous range into the 0-10 zone, whereas the Fisher F75 has 0-15 as ground/ferrous. In general, having more numeric range devoted to the non-ferrous portion of the scale is advantageous for jewelry hunters. It allows more room to separate a certain pesky aluminum tab from possible gold items.

The new Nokta Impact is quite unique in offering the choice of either 0-15 ferrous or 0-40 ferrous out of 100. Relic hunters or anyone just wanting to separate ferrous from non-ferrous tend to prefer the expanded ferrous range.

 

[Steve Herschbach]

Here is a chart with some of the various items that can fall into the various White's VDI ranges. Click for larger version.

[Steve Herschbach]

I did up my own chart for the Minelab X-Terra 50 back when I had one. It in my opinion gives a good sense of the order in which common items without being too complicated. Note that the X-Terra 50 used a VDI scale running from -9 to 0 to +45, so it is compressed into a tighter range compared to the White's or Nokta/Makro scales. But again, the order in which items are found remains the same.

Tone     VDI  Items

Very Hi .45 Steel Halves, Dollars
Very Hi .42 Quarters, Large Silver Rings
High .... 39 Silver Rings
High .... 36 Penny/Dime, Small Silver Rings
High .... 33 IH Penny
High .... 30 Zinc Penny, IH Penny
High .... 27 Screw Cap, IH Penny, Large Aluminum
Medium 24 Heavy Square Tabs, $5 Gold, Very Lg Men's Rings
Medium 21 Large Pull Tabs, Large Men's Rings
Medium 18 Pull Tabs Men's Rings
Medium 15 Small Pull Tabs, Erasers, Small Mens Rings
Medium 12 Light Square Tabs, Nickels, Erasers, Beavertails, Lg Women's Rings
Medium 09 Beavertails, Heavy Foil, Erasers, Med Women's Rings
Medium 06 Medium Foil, Small Women's Rings
Medium 03 Light Foil, Small Jewelry
Low ..... -3 Wire, Pins, Very Small Jewelry
Low ..... -6 Nails
Low ..... -9 Hot Rock, Large Iron

Notes:  45 is more often a steel junk indication than the very rare dollar or half. Mens rings fall mostly into 21 followed by 24. Womens rings are heavy in 6 and 9 followed by 12. 18 is the heavy pull tab range and sparse on rings (too high for most women's rings, too low for most mens). 15 also has fewer rings but also less junk. All these observations are only true for my area and mix of targets and so must be taken with a large grain of salt, are are only intended as an aid to those just starting out. You can get junk in any segment, and good finds in any segment!

Here is a simplified version, a combination of most likely targets and "wishful thinking". 21 is more likely to be a large pull tab, but it is the hottest number for men's rings, at least out of my collection. 18 might be a ring, but fewer fall there than in lower or higher numbers, and it is very heavy in common pull tabs.

45 Steel
42 Quarter
39 Silver
36 Penny/Dime
33 IH Penny
30 Zinc Penny
27 Screw Cap
24 Large Men's Ring
21 Men's Ring
18 Large Tab
15 Small Tab
12 Nickel
09 Womens Ring
06 Small Women's Ring
03 Foil
-3 Wire
-6 Nails
-9 Hot Rock

[Steve Herschbach]

Here is the final thought for the night. Back when Fisher did the CZ series of detectors they did lots of ring testing. They tested 255 rings for the CZ-70 and came up with some interesting percentages. I have seen these percentages reflected in the field.

2% of the rings were in the copper penny, dime, quarter range.

4% of the rings were in the zinc penny/screw cap range

49% of the rings were in the "pull tab" range above U.S. nickel

10% of the rings were in the nickel range

36% of the rings were in the foil range below nickel

0% of the rings were in the iron range

Now look at my simplified X-Terra VDI chart in the post immediately above. What we are seeing is the difference between women's rings and men's rings. Women's rings tend to be small high quality rings, and cluster in the foil range. They are the rings most likely to have stones and be of high value. Men's rings are significantly larger and heavier, and fall into the area above U.S. nickel. There is a weak spot or gap between women's and men's rings in the nickel range. This is of course a gross generality but I have found it to be true in my own detecting. You can use this to good effect when looking at an area and deciding what might be found there. Is the area more likely to hold men's rings and women's rings? The football field will more likely have men's rings. The shallow childs wading area or tot lot will lean women's rings. I do a lot of heavy surf detecting, and nearly all my finds are men's rings.

I have mentioned the Fisher CZ detectors. Rumor has it that CZ stood for "Coin Zapper". The CZ detector are unique in having a shuffled discrimination scale that puts nickels up high with the other coins. Little attention is paid to the fact they also lumped the bulk of the ring zones together. In addition to visual target identification, the CZ  has 3-tone, audio target ID. A low tone is for iron, a medium tone Is for pull tabs and foil and a high tone is for coins. A fourth tone, which sounds like a telephone, alerts you to large, shallow targets that are usually (but not always!) trash.

Basically with a CZ, low tone is iron, medium tone the ring range, and high tone the coin range. The newest model, the CZ-3D adds a fourth tone to the scheme designed to capture old coins in the zinc penny range, but for jewelry detecting the original CZ scheme is pretty simple and ingenious.

The good news is with newer detectors that have custom tone id ranges you can duplicate this setup yourself. Fisher also offers this modified tone scheme in the F75 models. The main thing is to be aware, no matter what detector you are using, or where the possible ring "hot spots" are on the VDI scale depending on where you are hunting.

From Fisher CZ-70 Pro Owners Manual, page 24 (tones added):

 

[Steve Herschbach]

OK, what follows is not a recommendation. It is simply what I tend to do. Others have other ideas.

More often than not when park hunting for jewelry I focus first on deciding on a good site. For me that means anywhere hands are in action making tossing, pushing, or flinging motions. Soccer fields and children's swings are classic examples. If I think the site is good, I simply dig all non-ferrous targets.

However, if time is limited and my patience thinner than normal I might do two other things. If I really am just hot to chase gold I may very well just knock out the entire high end coin range except for quarters. Seems like lots of quarters out there these days and they add up fast. You might question the dollar area but that is normally large junk.

Dollar.....................Reject
Quarter..................Accept
Penny/Dime...........Reject
IH Penny................Reject
Zinc Penny.............Reject
Screw Cap.............Accept
Large Men's Ring...Accept
Men's Ring.............Accept
Large Tab...............Accept
Small Tab...............Accept
Nickel.....................Accept
Womens Ring.........Accept
Sm Womens Ring...Accept
Foil..........................Accept
Wire........................Reject
Nails........................Reject
Hot Rock.................Reject

Again, it is normally just dig all non-ferrous targets. But if cherry picking I may skip most coins, especially zinc pennies. Yes, rings could appear there, but you have to call your shots as you please. So with that in mind if my time is really limited and I really have no patience, I may go to my final cherry pick mode. Skip the quarters also, and the screw caps, and the very light foil. There can be lots of light foil out there and the gold you might find there will be tiny stuff, like ear rings or thin chains.

Dollar.....................Reject
Quarter..................Reject
Penny/Dime...........Reject
IH Penny................Reject
Zinc Penny.............Reject
Screw Cap.............Reject
Large Men's Ring...Accept
Men's Ring.............Accept
Large Tab...............Accept
Small Tab...............Accept
Nickel.....................Accept
Womens Ring.........Accept
Sm Womens Ring...Accept
Foil..........................Reject
Wire........................Reject
Nails........................Reject
Hot Rock.................Reject

That zone is where you are going to find the majority of gold jewelry items by weight. If in an area where men's rings are a big factor, then opening up the screw cap and even zinc penny range makes sense. Where women's jewelry is prevalent keeping that light foil area open makes sense. Conversely, if you get into a place where one certain type of pull tab is driving you crazy, you may want to knock it out.

There are no clear answers here. It is all about time, patience, and calling the odds based on your experience. There are other strategies, like just digging nice, clean, solid sounding signals. I tend to always stick with shallow targets that require nothing more than a pinpointer and screwdriver to recover, as plugging takes too much time. Recovering huge volumes of targets matters, and plugging causes too much damage if done every foot. My standard procedure is just pop out any non-ferrous target my pinpointer can hit. If my pinpointer can't detect it it is too deep and so I move on to the next target. And as noted above I may vary my disc settings to suit my mood or circumstances.

Great books to help you out from Clive Clynick at http://www.clivesgoldpage.com/

 

[Mark Gillespie]

Very impressed with your compilation of information Steve, thanks, yet again.