I ran a quick horary astrology chart to see if I could get any insights from it on this. I used Rex Bills' Rulership Book to come up with correspondences. The key points that came out of the chart were:
The earring is ruled by Venus (jewelry) and Venus is in Aries (ruler of diamonds), so Venus provides the significator for the lost item.
Venus is disposited by Mars, which is in opposition to Venus. The earring is probably nowhere near where you first lost it (but then, you've already found that out).
Aries rules the east, and the east wall of a room. It also rules "less-frequented places."
Venus is located exactly on the cusp of the Regiomontanus 12th House, which is also an "eastern" location in the chart. The 12th House rules hidden things and remote or secluded places. This echoes Aries' rulership of "less-frequented places."
Did you actually hear the earring fall when you lost it? Possibly it's snagged on a piece of clothing hanging in a closet at rhe east side of the room. Some astrologers give clothing rulership to Venus, but most seem to assign it to Mercury, which in this chart is in the 1st House in Gemini, the sign of its rulership, and Gemini is on the Ascendant - another "eastern" pointer. Venus is very closely sextile to Mercury, lending some credibility to this theory. Or you may have accidentaly kicked it against the east wall and it may be hiding under the edge of a rug.
Another way to look at this is by taking the Lord of the 2nd House (personal property). Here, Cancer is on the 2nd House cusp and its ruler, the Moon, is in Taurus (sign associated with jewelry). The Moon is "imprisoned" in the 12th House, and its sign (Taurus) is also intercepted in the 12th. The Moon is also considered a "secondary" significator of lost items, but, since it reinforces the indications given by Venus, we don't need to spend much time on it. I do notice that the last aspect made by the Moon is a sextile to Jupiter, which is in Cancer, sign of its exaltation, and in the 2nd House of personal property. Looks to me like the earring will be found after much diligent searching and a good bit of luck. (But the point about Jupiter could also just mean that it's a very valuable piece.)
I would say, though, that with both significators in the 12th House, the earring is very well and thoroughly concealed. Alternatively, the 12th rules "losses," so the item may indeed be irretrievable. But I don't think so.
ETA: A few things I missed mentioning. Cancer rules "wet places" and "places near water," as well as the north. The hypothetical clothing with earring embedded may now be with the laundery at the north end of the house, or the earring may have come off inside the washing machine. Also, according to John Frawley, since the significator is in a Fire sign the earring could be near a heating register/radiator or "near the walls." He also gives clothing (especially women's) to Venus, as well as bedding or soft furnishings, so look in the furniture as well. Finally, Frawley says connections to Mercury indicate it could be near or with "books or knick-knacks." Since Venus is on the cusp of the 12th, the earring could also be near the door of the room indicated by that house (Frawley says "garage" or "junk room;" I might say "closet.")
ETA: Uh oh. I think I need to take a half-step back here regarding the likelihood of finding the earring. Venus is conjunct the Moon's South Node within 3 minutes of arc. In traditional horary, a significator conjunct the South Node was considered unlucky for the outcome. In this case it is past the conjunction (separating), so it's influence may be less pronounced. But I think it would make the finding that much more challenging.
