Shilajit is a blackish brown rock exudate that is found in some mountain ranges in the 1000-5000m height range, and known to be cultured in the Himilayan and Hindukush ranges, used in Ayurveda medicine as a vitality enhancer and adaptogen compound with historically 'miraculous' effects. Possibly because of these effects reported to it, it was named Shilajit; Shilajit translates from Sanskrit means Conqueror of mountains and destroyer of weakness.
Shilajit may also consist of plant microbial metabolites that occur in rock rhizospheres. One study assessing components of Shilajit noted that two metabolites present in the sample also existed in rhus succedanea and Piszacia inregerrima, two plants that grow in the same region the sample was derived from, the Kumaon Hills.
Shilajit tends to contain:
Fulvic Acid, seen as the main bioactive component
Di-benzo-alpha-pyrones (DBPs) and the chromoproteins associated with DBPs; several other compounds such as 3,8-dihydroxy-dibenzo-α-pyrone that belong to the Dibenzo-α-pyrone class of molecules
24(Z)-3/3-hydroxytirucalla-7,24-dien-26-oic acid, a triterpenoic molecule
Glycine, Aspartic Acid, and Glutamic Acid in minor quantities
Fulvic acid is probably the most relevant molecule of interest, as many modern supplements are standardized for Fulvic acid. Tirucalloic-type triterpenoids appear to also be relevant. Shilajit possesses a good deal of variability naturally based on growing conditions
Shilajit has been reported to have a bitter taste slightly remiscent of very pure chocolate and a scent reminiscient of cow urine. Its smell has also has been compared to bitumen. 
Shilajit has been tested in a preliminary in vitro study using Daphnia, which are highly responsive to added pharmaceuticals to culture; it was found that concentrations of 1-100ppm had a negative chronotropic effect, reducing heart rate frequency by 7.65%, 15% and 28.45% at 1, 10, and 100ppm respectively and exerting a positive chronotropic effect at 1000ppm. Explanations for the observed effects are currently unknown.
One study investigating 2g Shilajit daily (6.61% Fulvic Acid) noted that (in otherwise healthy adults) that Shilajit was able to reduce triglycerides by 21.7%, LDL-C by 22.6%, vLDL-C by 20.6%, and increase HDL-C by 5.8%.
Shilajit was found to not interact with either glutamate receptors nor acetylcholine (nAchR) receptors in vitro.
Shilajit is thought to be able to influence the perception of pain (nocioreception) due to apparent pain reduction in a hot-water tail flick test in mice and a later study noting that Shilajit was able to attenuate the tolerance of morphine's pain-killing effect in mice injected with 0.1mg and 1mg/kg Shijalit, although this latter study failed to find analgesic effects of Shilajit per se.
In isolated Substantia Gelatinosa (SG) neurons from the Trigeminal Subnucleus Caudalis (involved in nocioreception) of mice, Shilajit caused repeated inward currents in neurons (thought to be mediated post-synaptically) in a concentration-dependent manner up to 1mg/mL and as low as 100ug/mL, with an estimated EC50 of 562 µg/ml. These effects were abolished with 2uM strychnine (glycine receptor antagonist) and partially reduced by a GABAA antagonist.
Some interactions with pain in the direction of reducing the perception of pain, but efficacy is unknown
Currently, one rat study using a polyherbal Ayurvedic formulation that includes Shilajit noted improvements in learning and memory acquisition but is too confounded to determine the efficacy of Shilajit per se.
One human intervention with 200mg (54.8% Fulvic Acid) Shilajit daily for 90 days was associated with a decrease blood glucose by 6.8% relative to baseline in otherwise healthy males relative to their own baseline. A reduction in blood glucose was not observed with 2g (6.61% Fulvic Acid) Shilajit over 45 days in otherwise healthy persons.
Two human studies, differing results with different doses. Too preliminary to say anything
One in vitro study assessing mast cell degranulation noted that three isolated components of Shilajit, including Fulvic acids, were able to significantly reduce degranulation of mast cells in the presence of stimuli, suggesting anti-allergic effects.
In otherwise normal rats, Shilajit at 25, 50, or 100mg/kg daily for 6 weeks found dose-dependent increases in sperm count in the testes and epididymus up to 130.4% and 378%, respectively at the 100mg/kg dose, independent of changes in testicular weight.
One intervention in men deemed oligospermic (low sperm count) given 200mg Shilajit daily of 54.8% Fulvic Acid (two divided doses with meals) for 90 days noted improved sperm parameters including volume (37.6%), count (61.4%), motility (12.4-17.4%), percent of sperm with normal morphology (18.9%) coupled with a reduction in lipid peroxidation levels (MDA) in the semen (18.7%).
In female rats fed 25, 50, and 100mg/kg Shilajit daily for 6 weeks, Shilajit was associated with more rats in estrus and thought to have stimulating properties on oocytes and ovulation frequency.
One human intervention with 200mg Shilajit (54.8% Fulvic Acid) daily for 90 days in infertile men was associated with an increase in testosterone by 23.5% (4.85ng/mL to 5.99ng/mL) in serum.
200mg Shilajit daily for 90 days in infertile men is associated with an increase in Follicle-Stimulating Hormone (FSH) by 9.8%, with no significant influence on Luteinizing Hormone (LH) in serum.
NR-ANX-C is a term used to refer to a polyherbal formulation of Ayurveda consisting of Ashwagandha, Holy Basil, Camellia Sinensis (source of Green Tea Catechins), Triphala, and Shilajit in a 2:2:4:2:1 ratio and ethanolic extracts of Camellia and Holy Basil, with water extracts of the other three components. In a model of Haloperidol-induced cognitive defici, 25mg/kg of this decoction was able to acutely and chronically (one week of supplementation) reduce the cataleptic score associated with Haloperidol by 14-39% of control, which was associated with antioxidative effects.
In rats, 100mg/kg bodyweight Shilajit daily for 6 weeks is not associated with any abnormal organ weights relative to control.
One human intervention using 200mg daily in men for 90 days (dose determined from Ayurvedic recommendations) failed to note any clinically significant toxicological symptoms when measuring standard toxixological biomarkers, but noted a small decrease in serum creatinine by 7.8% and increases in both Haemoglobin (no morphological changes of RBCs) and WBC count by 5.2% and 6% respectively. A larger dose of 2g Shilajit (6.61% fulvic acid) daily for 45 days in humans aged 16-30yrs did not note any significant toxicological signs in serum, but did not note any significant influence on haemoglobin.