Drachms were the most important coins for the Parthians and they were thus the most commonly minted. For that very reason, the numbers of extant examples on the market, I have abandoned them. I hope to sell off most of my drachms in the next few years - and in fact have slowly begun that process. Where Parthians are concerned, I am currently only after the more challenging game of the silver and billon tets and the fractions. (I don't collect Parthian bronzes)
On the occasion of my picking up only my 10th Parthian fraction, I thought I'd provide some information about these issues, and post my group. Some of mine have been shown here before (but never as a group), so please forgive the unrequested encore. The few short paragraphs I've written, below, literally constitute more information about Parthian fractions than I've ever seen compiled online...there's a serious dearth of available information about them.
Parthian silver fractions consist of
obols (typically around 11mm, approximately 0.6 gm, and equivalent to 1/6 of a drachm or 8 chalkoi, the base metal coins used for "small change"),
diobols (around 12mm, approximately 1.4 gm), and
hemidrachms or
triobols (of which I own none; around 14mm, approximately 2 gm, equivalent to three obols or half a drachm). While the 0.6 gm standard for the obols seems to have been tightly monitored and respected (most seem to be within +/- 0.1 gm of that standard), there is considerable variance in the weights of the diobols, which can deviate as much as 0.5 gm from their 1.4 standard. Hemidrachms/triobols may vary as well, but typically no more than +/- 0.2 gm from their 2 gm standard.
Parthian fractions carry on the iconographic tradition of the drachms. That is, obverses feature the bust of the current (issuing) king wearing a bashlyk or diadem (earlier issues) or tiara (later issues), with a seated archer reverse. The notable exception to the rule for reverses is the Mithradates I obol (Sellwood types 12.4 and 12.5), which features a bust of the king's brother Bagasis for the reverse.
While Parthian drachms are generally common and tetradrachms fairly common (depending on the type, of course), the fractional silver denominations are harder to come by since they were minted in significantly smaller quantities. Thus all Parthian silver fractions are considered scarce or rare - although exactly how rare depends on the particular issue.
The initial fractional issue was a hemidrachm/triobol of Parthia's first king, Arsakes I, 247 - 211 BC. After Arsakes' rule, fractions were only issued intermittently by some kings, and only up to the reign of Pakoros II (78 - 105 AD). During that span of intermittent minting of fractional denominations there was a period of more than a century, between the reigns of Orodes II (57 - 38 BC) and Vologases I (51 - 78 AD), that no obols, diobols, or hemidrachms were issued at all. For the remaining 120 or so years of the empire's existence after Pakoros II, no silver fractions seem to have been issued.
Many Parthian fractions are identified in David Sellwood's
The Coinage of Parthia (2nd edition, 1980). However, nine years after the publication of the second (and last) edition of that book, Sellwood documented seven additional types based on a new (early 1980s) hoard from the Iranian province of Fars, an area that in antiquity had belonged to Persis. Persis was among the group of semi-autonomous kingdoms located in the territories controlled by Parthia. All of these newly discovered Parthian fractions were minted in Persis between the mid-first century and early second century AD. Sellwood's catalog of the new discoveries was published in 1989, in the
Numismatic Chronicle #149.
Prices for Parthian silver fractions vary greatly based on type, condition, degree of rarity and, of course, provenance, but many can be picked up for under $200 - and some significantly lower than that. I got my holed Mithradates I obol below, for example, for just $30. (One from the same set of dies, however, is currently included in CNG Sale 105 with an estimate of $300) My winning bid for the Orodes II obol, below, was $60 - pretty low considering its R1 rarity rating from Shore. The chipped Mithradates I obol with the right-facing bust - which was purchased directly from Pieter van't Haaff (author of the
Catalogue of Elymaean Coinage) and is from his collection - was $70. Formerly this same coin was in the important Parthian collection of Robert Gonnella. It is documented at Parthia.com as PDC 39892. It's not a pretty coin, but the price was decent given both the rarity of that issue and the important provenance. Some of the other coins below were, unfortunately for me, much more expensive.
Due to the relatively low output of the fractions - and low numbers of extant examples - many are documented on study sites (ACSearch.info, Coinproject, Parthia.com, etc.). In addition, a number have passed through important Parthian collections at one point or another, such as the collections of Todd Ballen, David Sellwood, Robert Gonnella, Pieter van't Haaff, Bellaria, Nisa, etc. I'll add to this list Parthicus, a member of Cointalk, whose extensive Parthian collection was just auctioned off. So the chances of owning a provenanced coin - or of having the nice surprise of chancing upon your coin posted online somewhere - is pretty decent with these.

Mithradates I (Sellwood and Shore); Phriapatius to Mithradates I (Assar)
obol, 171 - 138 BC
Hekatompylos mint
Sellwood 9.7

Mithradates I (Sellwood and Shore); Phriapatius to Mithradates I (Assar)
obol, 171 - 138 BC
(0.62 gm)
Hekatompylos mint
Sellwood 10.19
Ex-David Sellwood Collection
Gotta love the nose!

Mithradates I
obol, 171 - 138 BC
(9 mm, 0.49 gm)
Ecbatana mint
Sellwood 12.4
(The reverse here depicts Mithradates' brother Bagasis)
Ex-Van't Haaff Collection; Ex-Dr. Robert Gonnella Collection

Orodes II
obol, 57 - 38 BC
(10 mm, 0.56 gm)
Ecbatana mint
Sellwood 48.15; Sunrise 385; Shore 265
Ex-Parthicus Collection

Vologases I
diobol, 51 - 78 AD
(1.22 gm)
Probably minted in Persis, which at the time was a subject kingdom of Parthia
Sellwood "New Parthian Coin Types," Numismatic Chronicle 149, 1989, type 1; Shore 379
This coin also listed at Parthia.com as PDC 8262

Vologases I
diobol, 51 - 78 AD
(13 mm, 1.19 gm)
Probably minted in Persis, which at the time was a subject kingdom of Parthia
Sellwood "New Parthian Coin Types," Numismatic Chronicle 149, 1989, type 2; Shore 381
Ex-David Sellwood Collection

Vologases I
diobol, 51 - 78 AD
(13 mm, 1.23 gm)
Probably minted in Persis, which at the time was a subject kingdom of Parthia
Sellwood "New Parthian Coin Types," Numismatic Chronicle 149, 1989, type 2; Shore 381
Ex-Parthicus Collection

Vologases II
diobol, 77 - 80 AD
(1.44 gm)
Probably minted in Persis, which at the time was a subject kingdom of Parthia
Sellwood "New Parthian Coin Types," Numismatic Chronicle 149, 1989, type 3 variant (archer facing the wrong way! LOL!)
This coin listed at
https://www.acsearch.info/search.html?id=1832576 and
https://www.acsearch.info/media/ima...832576.m.jpg
Pakoros II
diobol, 78 - 105 AD
Despite the Ecbatana monogram, probably minted in Persis, which at the time was a subject kingdom of Parthia
Sellwood "New Parthian Coin Types," Numismatic Chronicle 149, 1989, type 6; Shore 402
This coin also listed at
http://www.acsearch.info/record.html?id=560213 and
http://www.acsearch.info/images/57/560213.jpg
Pakoros II
diobol, 78 - 105 AD
Despite the Ecbatana monogram, probably minted in Persis, which at the time was a subject kingdom of Parthia
Sellwood "New Parthian Coin Types," Numismatic Chronicle 1989, type 6; Shore 402