Really depends on where you plan to hunt for the jewelry at. In beach sand I've heard prospecting units work fine.
For the inland sites I hunt which include athletic fields, parks, schools and playgrounds I have two recommendations: Fisher F5 with the stock 10" elliptical coil and Whites V3i. Both of these models can run hot at low gain settings which is very important when hunting small jewelry items in turf and they both have features that enhance a jewelry recovery in trash. The V3i is the more feature rich of the two, but the F5 does real well at this even with its 7.8 kHz operating frequency.
High frequency is often touted as better for small jewelry but the issue with high frequency units is that they scream on every little bit of non-ferrous trash. For small jewelry targets in non-ferrous trash you need to be able to key in to the small, solid sounding signals. The F5's 7.8 kHz can give you some definition between solid and non-solid, where as something operating in the 19kHz range cannot and it's ability to enhance the audio of the small signals is a big plus.
Besides the basic requirement of low gain with high sensitivity, the V3i has the ability to show you how the target responds in multiple frequencies. The high 22.5 kHz will scream on the tiny target but the low 2.5 kHz frequency will tell you if the target has mass. It also has multiple ways to view, enhance and focus on the targets.
Why low gain and high sensitivity? In turf, all the small stuff is in the grass roots. Surface to maybe 4" tops. The more you raise your gain the more targets and ground noise you have to deal with. You start hearing whats below your desired target depth and more masking goes on. Models that separate the Gain and Threshold (sensitivity) allow you to keep your gain low, but yet still be sensitive to the small targets. The other benefit to low gain settings is that your coil footprint is kept small so you only hear whats under your coil, not the stuff on the sides. The surface trash isn't falsing and blocking your view.
Tom Dankowski has a good video for beach hunting where in one segment he discusses micro jewelry hunting on the beach with a Fisher GoldBug II, which is a gold prospecting unit that runs at 71 kHz. If I lived near a beach, I'd use one of them
treasure jewelry. Have also heard the Gold Racer does good on beach sand for micro jewelry as well.
There is a difference between Micro and Small. Micro requires very high frequency. Small does not. the dividing like is blurred, depending on who you ask. If the F5 with a Gain of 5 and a Threshold of +5 won't give a digable signal a inch or two from the coil, I call that Micro.
Hope that helps,
HH
Mike