Table 1: Characteristics of most common crystals and foreign bodies in synovial fluid or synovium*.
| Crystal [Chemical formule] | Breaking direction (Elongation) | Breaking intensity | Shape | Size |
| Monosodium urate (MSU)-[C5H4N4O3]- Figure 2 | negative | intensive | needle,rod, sperule | Submicroscopic -40 μm 5-25 μm |
| Calcium pyrophosphate dihydrate (CPPD) [Ca2P2O7.2H2O]- Figure 3 | positive | strong | rod, rhomboid | < 40 μm |
| Calcium hydroxyapatite – HA [Ca5(PO4)3(OH)], [Ca10(PO4)6OH2] or [Ca5(PO4)3OH.2H2O] **- Figure 4 | positive | weak | rod, clusters: shiny coins | individual crystals: submicroscopic [1] 50-500 nm [2] clusters of crystals: 1-5 μm [2] 1.9-15.6 μm [3] |
| Octocalcium phosphate-[Ca8H2(PO4)6.5H2O] | ND | ND | ND | ND |
| Tricalcium phosphate-[Ca3(PO4)2] (whitelockite) | ND | ND | ND | ND |
| Dicalcium phosphate dehydrate [Ca2(PO4).2H2O] | ND | ND | ND | ND |
| Calcium hydrogen phosphate dihydrate-[CaH(PO4).2H2O] (orthophosphate, brushite) | positive | moderate | rod | 1-2 μm |
| Calcium oxalate [CaC2O4.H2O] | positive | variable | tetrahedron, rod | 1-2 μm |
| Cholesterol- [C27H46O]- Figure 5 | negative and/or positive | variable | rhomboidal, notched, needle-shaped cloven -separate or clusters | 5-40 μm [3] |
| Liquid lipid crystals- Figure 5 | Maltese cross, positive | variable | spherules | 0.5-30 μm [3] |
| Lithium heparin | positive | weak | polymorphous | 2-5 μm |
| Corticosteroid | variable | mostly strong | polymorphous | 1-40 μm |
| Talc | Maltese cross, negative | strong | ovoid | 1-40 μm |
| Glass fiber | positive | strong | orbed-oval | variable |
| Surgical sutures | negative | strong | clusters | variable |
| Methylmetacrylate | intensive | fragments | variable | |
| Metallosis (swarf) | none | none | spotty deposits | variable |
| Hemosiderin crystals | Maltese cross, positive | weak | groups | 5-40 μm |
| Osmium | none | none | spotty deposits | variable |
| Ochronosis (artifact) colored refractile collagen fragments | positive | variable | fragments | variable |
*Modified from Gatter and Schumacher [3] and from Gardner and McClure [4].
ND- no data; **the terminology of hydroxyapatite is not uniform in the pertinent literature; the formula of HA is: [Ca5(PO4)3(OH)] or [Ca10(PO4)6OH2].